Support flat config

[Y0g44]

This is a pull request to support flat config for eslint-config-airbnb-base and eslint-config-airbnb and do some rewriting.

Example:

// eslint.config.cjs
const airbnb = require('eslint-config-airbnb/flat');

module.exports = [
  ...airbnb,
  // ......
];

// eslint.config.mjs
import airbnb from 'eslint-config-airbnb/flat';

export default [
  ...airbnb,
  // ......
];

[Y0g44]

By the way, I haven't fully tested it yet. So I'm not sure if it will really work.

[ljharb]

I’m confused, what is the “base” folder? The existing files directly inside rules will remain where they are, and shouldn’t be duplicated.

[ljharb]

Why was this closed and replaced with a duplicate #3081?

[Y0g44]

I tried updating, I'm new to github so I don't really understand how this works. Sorry.