Implement localized week information (#1454)

* begin adding support for locale-dependent start of week

* Use an options parameter for locale week startOf

* Add tests for locale week start and end

* Add useLocaleWeeks option to DateTime.hasSame and add tests

* Add useLocaleWeeks option to Interval.count

* Add Intl.Locale.weekInfo as a reported optional feature and test for it

* Add Info.getStartOfWeek method

* Add DateTime.isWeekend and DateTime.localDayOfWeek getters

* Improve localized week code in DateTime#startOf

* Implement DateTime#localWeekYear and DateTime#localWeekNumber
Adjust localWeekDay to be in line with weekday

* Add documentatino for DateTime#localWeekNumber and DateTime#localWeekYear

* Add Info.getMinimumDaysInFirstWeek and Info.getWeekendWeekdays and tests

* Add DateTime#isWeekend test

* Add tests for DateTime#localWeekNumber and localWeekYear

* Add tests for localWeekday

* Add formatting tokens for localWeekYear and localWeekNumber

* Add tests for formatting tokens n, nn, ii, iiii

* Implement DateTime#weeksInLocalWeekYear and proper week year length calculations based on locale week settings

* Support setting locale based week units with `DateTime#set`

* Fix Typo

* Add tests for DateTime#set with locale based week units and DateTime#weeksInLocalWeekYear

* Accept locale-based week units in DateTime#fromObject

* Add Settings.defaultWeekSettings

* Add documentation for locale-based weeks

* Remove unused value from usesLocalWeekValues

* Remove incorrect claim in documentation

* Remove dumb import

docs/formatting.md

@@ -177,7 +177,7 @@ The macro options available correspond one-to-one with the preset formats define
 (Examples below given for `2014-08-06T13:07:04.054` considered as a local time in America/New_York).
 
 | Standalone token | Format token | Description                                                    | Example                                                       |
-| ---------------- | ------------ | -------------------------------------------------------------- | ------------------------------------------------------------- |
+|------------------| ------------ |----------------------------------------------------------------| ------------------------------------------------------------- |
 | S                |              | millisecond, no padding                                        | `54`                                                          |
 | SSS              |              | millisecond, padded to 3                                       | `054`                                                         |
 | u                |              | fractional seconds, functionally identical to SSS              | `054`                                                         |
@@ -219,6 +219,10 @@ The macro options available correspond one-to-one with the preset formats define
 | kkkk             |              | ISO week year, padded to 4                                     | `2014`                                                        |
 | W                |              | ISO week number, unpadded                                      | `32`                                                          |
 | WW               |              | ISO week number, padded to 2                                   | `32`                                                          |
+| ii               |              | Local week year, unpadded                                      | `14`                                                          |
+| iiii             |              | Local week year, padded to 4                                   | `2014`                                                        |
+| n                |              | Local week number, unpadded                                    | `32`                                                          |
+| nn               |              | Local week number, padded to 2                                 | `32`                                                          |
 | o                |              | ordinal (day of year), unpadded                                | `218`                                                         |
 | ooo              |              | ordinal (day of year), padded to 3                             | `218`                                                         |
 | q                |              | quarter, no padding                                            | `3`                                                           |

docs/intl.md

@@ -148,3 +148,71 @@ Similar to `locale`, you can set the default numbering system for new instances:
 ```js
 Settings.defaultNumberingSystem = "beng";
 ```
+
+## Locale-based weeks
+
+Most of Luxon uses the [ISO week date](https://en.wikipedia.org/wiki/ISO_week_date) system when working with week-related data.
+This means that the week starts on Monday and the first week of the year is that week, which has 4 or more of its days in January.
+
+This definition works for most use-cases, however locales can define different rules. For example, in many English-speaking countries
+the week is said to start on Sunday and the 1 January always defines the first week of the year. This information is
+available through the Luxon as well.
+
+Note that your runtime needs to support [`Intl.Locale#getWeekInfo`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/Locale/getWeekInfo) for this to have an effect. If unsupported, Luxon will fall back
+to using the ISO week dates.
+
+### Accessing locale-based week info
+
+The `Info` class exposes methods `getStartOfWeek`, `getMinimumDaysInFirstWeek` and `getWeekendWeekdays` for informational
+purposes.
+
+### Accessing locale-based week data
+
+```js
+const dt = DateTime.local(2022, 1, 1, { locale: "en-US" });
+dt.localWeekday // 7, because Saturday is the 7th day of the week in en-US
+dt.localWeekNumber // 1, because 1 January is always in the first week in en-US
+dt.localWeekYear // 2022, because 1 January is always in the first week in en-US
+dt.weeksInLocalWeekYear // 53, because 2022 has 53 weeks in en-US
+```
+
+### Using locale-based week data when creating DateTimes
+
+When creating a DateTime instance using `fromObject`, you can use the `localWeekday`, `localWeekNumber` and `localWeekYear`
+properties. They will use the same locale that the newly created DateTime will use, either explicitly provided or falling back
+to the system default.
+
+```js
+const dt = DateTime.fromObject({localWeekYear: 2022, localWeekNumber: 53, localWeekday: 7});
+dt.toISODate(); // 2022-12-31
+```
+
+### Setting locale-based week data
+
+When modifying an existing DateTime instance using `set`, you can use the `localWeekday`, `localWeekNumber` and `localWeekYear`
+properties. They will use the locale of the existing DateTime as reference.
+
+```js
+const dt = DateTime.local(2022, 1, 2, { locale: "en-US" });
+const modified = dt.set({localWeekNumber: 2});
+modified.toISODate(); // 2022-01-08
+```
+
+### Locale-based weeks with math
+
+The methods `startOf`, `endOf`, `hasSame` of `DateTime` as well as `count` of `Interval` accept an option `useLocaleWeeks`. 
+If enabled, the methods will treat the `week` unit according to the locale, respecting the correct start of the week.
+
+```js
+const dt = DateTime.local(2022, 1, 6, { locale: "en-US" });
+const startOfWeek = dt.startOf('week', {useLocaleWeeks: true});
+startOfWeek.toISODate(); // 2022-01-02, a Sunday
+```
+
+### Overriding defaults
+
+You can override the runtime-provided defaults for the week settings using `Settings.defaultWeekSettings`:
+
+```js
+Settings.defaultWeekSettings = { firstDay: 7, minimalDays: 1, weekend: [6, 7] }
+```