[feature request] add zh as a locale in accordance with window.navigator.languages

[ZYinMD]

In my app, I use window.navigator.languages to decide which locale to use to render relative date for my users. One thing I noticed is that zh is a valid locale in browsers, but not available in dayjs. It's an issue because I'm forced to pick a locale between zh-cn, zh-tw, and zh-hk for the user, and I don't know which to pick.

To see zh in Chrome, go to the ⋮ menu -> Settings -> Advanced (on the left sidebar) -> Languages -> Add languages -> Add Chinese. Then you'll see zh appear in your navigator.languages.

There are already other cases where a generic locale and its sub locales co-exists in dayjs, for example, es, es-us, es-do; fr, fr-ca, fr-ch, so I think adding a zh is completely reasonable.

I'd be happy to submit a PR if this can be agreed upon.

[iamkun]

搞

[iamkun]

fixed