Support flat config for eslint-config-airbnb-base

[tamiroh]

This pull request supports flat config for eslint-config-airbnb-base.

An example of usage is as follows.

// eslint.config.mjs

import airbnbBase from "eslint-config-airbnb-base/flat";

export default [
  ...airbnbBase,
  // Add your own configs
];

[mcorbelli]

Is this pull request fully functional? only approval is missing?

[ljharb]

@mcorbelli no, it’s not functional until eslint-find-rules is updated.

[KonRatt]

@ljharb FYI: eslint-find-rules does have flat-config support now: https://github.com/sarbbottam/eslint-find-rules/releases/tag/v4.2.0

[ljharb]

Thanks, I’m aware since I’m the one who published it :-)

[fazzabi]

The goal of this PR is to ensure compatibility between eslint-config-airbnb-base and ESLint 9, correct? It looks like you'll need to update line 83 to include ESLint 9 as a peer dependency.