Packages: Reimplement ESLint config as plugin

[aduth]

This pull request seeks to rename and repurpose the current (yet-unpublished) eslint-config package to an eslint-plugin package. This will allow the package to serve a dual-purpose of providing configurations and custom rules, where the latter is not possible without being published as its own, separate ESLint plugin.

The change in usage is minimal, in regular usage amounting to:

	extends: [
-		'@wordpress/eslint-config',
+		'plugin:@wordpress/eslint-plugin/recommended',
	],

In the course of doing so, I had observed significant divergence, redundancy, and a lack of opportunity for granularity between the bundled rulesets. The proposed changes seek to remedy this by:

• Flattening configs/rules into their respective configs/ files
  • This will also help lend clarity when proper rules are included with the plugins, separate from what are called here as "rulesets" (specific configurations of rules)
• Consolidate the default, ESNext, and ES5 rulesets
  • The default ruleset is now esnext, which itself is an extension of es5
• Separate out jsx-a11y and react configurations
  • Enables easy usage of the JavaScript-specific rules without also opting in to an assumed React code environment
  • Note: Technically I think the purest approach here would not define eslint-plugin-react and eslint-plugin-jsx-a11y as dependencies of eslint-config, but rather assume them as optional peer dependencies. For the purpose of easing developer "Getting Started" experience, I opted to leave them as dependencies, with the primary downside being that they'd be installed whilst not necessarily used by a consuming project.

Testing Instructions:

Verify there is no difference in linting results for the current codebase.

npm run lint-js

[ntwb]

LGTM

I'd like to add one more config, from the widely used JSHint "shared preset"

https://github.com/WordPress-Coding-Standards/eslint-plugin-wordpress/blob/master/lib/configs/rules/jshint.js

Primarily the reason for this is to use this config as a "drop in" reeplacement for cores current JSHint config, this will allow core to drop from JSHint and switch to ESLint easily as part of #WP31823

Then, over time the @wordpress/eslint-plugin/recommended rules can be applied to the legacy codebase, as this will take considerable time IMHO.

[aduth]

I'd like to add one more config, from the widely used JSHint "shared preset"

Let's plan to consider this as a follow-on I think. This pull request introduces "new" rulesets, but they're really just pulled from what had already existed.

[gziolo]

• Note: Technically I think the purest approach here would not define eslint-plugin-react and eslint-plugin-jsx-a11y as dependencies of eslint-config, but rather assume them as optional peer dependencies. For the purpose of easing developer "Getting Started" experience, I opted to leave them as dependencies, with the primary downside being that they'd be installed whilst not necessarily used by a consuming project.

This is a common trade-off in the case when you provide default configuration. I think it's fine as you assume that the default one is going to be used in the majority of cases. I think there are bigger issues if you would like to extend on this. I had the same thought process when proposing test-e2e and in particular puppeteer dependency. What if someone is not interested in using such script? Fortunately those are devDependencies so they don't have any impact on production code.

[gziolo]

Nice refactoring. Great job!

There is one rule which is duplicated and can be removed, and one rule I wasn't sure about. Otherwise, it's good to go.

[gziolo]

array-bracket-spacing is already in ES5 config.

[gziolo]

Is 'no-var': 'error', automatically applied to es6 env? It's missing when you compare to the previous setup.

[aduth]

Eagle eye, @gziolo 😄 Fixed up both of these. Also ran through once again the prior config to see if there were other missing rules (there weren't, as best I could tell).