Support flat config

[9takoyakii]

Pull request for support flat config.

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,
  // ......
];

[ljharb]

Please do not file duplicate pull requests; if you want to change one, reuse it.

[ghost]

Ok, sorry I made a duplicate pull request