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
];