feat: add support for Flat Config

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.

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',
    },
  },
];
```

config/flat/react.js

@@ -7,7 +7,7 @@
  */
 module.exports = {
   settings: {
-    'import/extensions': ['.js', '.jsx'],
+    'import/extensions': ['.js', '.jsx', '.mjs', '.cjs'],
   },
   languageOptions: {
     parserOptions: {

config/react.js

@@ -6,13 +6,11 @@
  * if you don't enable these settings at the top level.
  */
 module.exports = {
-
   settings: {
     'import/extensions': ['.js', '.jsx'],
   },
 
   parserOptions: {
     ecmaFeatures: { jsx: true },
   },
-
 };

config/typescript.js

@@ -9,7 +9,7 @@
 // `.ts`/`.tsx`/`.js`/`.jsx` implementation.
 const typeScriptExtensions = ['.ts', '.cts', '.mts', '.tsx'];
 
-const allExtensions = [...typeScriptExtensions, '.js', '.jsx'];
+const allExtensions = [...typeScriptExtensions, '.js', '.jsx', '.mjs', '.cjs'];
 
 module.exports = {
   settings: {

examples/flat/eslint.config.mjs

@@ -14,7 +14,7 @@ export default [
       ecmaVersion: 'latest',
       sourceType: 'module',
     },
-    ignores: ['eslint.config.js', '**/exports-unused.ts'],
+    ignores: ['eslint.config.mjs', '**/exports-unused.ts'],
     rules: {
       'no-unused-vars': 'off',
       'import/no-dynamic-require': 'warn',

package.json

@@ -108,6 +108,7 @@
     "eslint": "^2 || ^3 || ^4 || ^5 || ^6 || ^7.2.0 || ^8"
   },
   "dependencies": {
+    "@nodelib/fs.walk": "^2.0.0",
     "array-includes": "^3.1.7",
     "array.prototype.findlastindex": "^1.2.4",
     "array.prototype.flat": "^1.3.2",