This change adds support for ESLint's new Flat config system.  It maintains backwards compatibility with eslintrc style configs as well.

To achieve this, we're now dynamically creating flat configs on a new `flatConfigs` export.  I was a bit on the fence about using this convention, or the other convention that's become prevalent in the community: adding the flat configs directly to the `configs` object, but with a 'flat/' prefix.  I like this better, since it's slightly more ergonomic when using it in practice.  e.g. `...importX.flatConfigs.recommended` vs `...importX.configs['flat/recommended']`, but i'm open to changing that.

Example Usage

```js
import importPlugin from 'eslint-plugin-import';
import js from '@eslint/js';
import tsParser from '@typescript-eslint/parser';

export default [
  js.configs.recommended,
  importPlugin.flatConfigs.recommended,
  importPlugin.flatConfigs.react,
  importPlugin.flatConfigs.typescript,
  {
    files: ['**/*.{js,mjs,cjs,jsx,mjsx,ts,tsx,mtsx}'],
    languageOptions: {
      parser: tsParser,
      ecmaVersion: 'latest',
      sourceType: 'module',
    },
    ignores: ['eslint.config.js'],
    rules: {
      'no-unused-vars': 'off',
      'import/no-dynamic-require': 'warn',
      'import/no-nodejs-modules': 'warn',
    },
  },
];
```

Closes import-js#29