feat: add flat config support

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',
    },
  },
];
```

Note: in order to fill a gap in a future API gap for the `no-unused-module`, this takes advantage of a *proposed* new API on the ESLint context, that currently only exists in a POC state (unreleased).

Closes import-js#2556

.eslintignore

@@ -8,3 +8,4 @@ test/fixtures/typescript-d-ts/
 # we want to ignore "test/fixtures" here, but unfortunately doing so would
 # interfere with unit test and fail it for some reason.
 # test/fixtures
+examples

examples/flat/eslint.config.mjs

@@ -0,0 +1,25 @@
+import importPlugin from 'eslint-plugin-import-x';
+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.mjs', '**/exports-unused.ts'],
+    rules: {
+      'no-unused-vars': 'off',
+      'import-x/no-dynamic-require': 'warn',
+      'import-x/no-nodejs-modules': 'warn',
+      'import-x/no-unused-modules': ['warn', { unusedExports: true }],
+    },
+  },
+];

examples/flat/package.json

@@ -0,0 +1,19 @@
+{
+  "name": "flat",
+  "version": "1.0.0",
+  "main": "index.js",
+  "scripts": {
+    "clean": "rimraf node_modules",
+    "lint": "cross-env ESLINT_USE_FLAT_CONFIG=true eslint src --report-unused-disable-directives"
+  },
+  "devDependencies": {
+    "@eslint/js": "^9.5.0",
+    "@types/node": "^20.14.5",
+    "@typescript-eslint/parser": "^7.13.1",
+    "cross-env": "^7.0.3",
+    "eslint": "^8.57.0",
+    "eslint-plugin-import-x": "link:../..",
+    "rimraf": "^6.0.1",
+    "typescript": "^5.4.5"
+  }
+}

examples/flat/src/exports-unused.ts

@@ -0,0 +1,12 @@
+export type ScalarType = string | number;
+export type ObjType = {
+  a: ScalarType;
+  b: ScalarType;
+};
+
+export const a = 13;
+export const b = 18;
+
+const defaultExport: ObjType = { a, b };
+
+export default defaultExport;

examples/flat/src/exports.ts

@@ -0,0 +1,12 @@
+export type ScalarType = string | number;
+export type ObjType = {
+  a: ScalarType;
+  b: ScalarType;
+};
+
+export const a = 13;
+export const b = 18;
+
+const defaultExport: ObjType = { a, b };
+
+export default defaultExport;

examples/flat/src/imports.ts

@@ -0,0 +1,7 @@
+//import c from './exports';
+import { a, b } from './exports';
+import type { ScalarType, ObjType } from './exports';
+
+import path from 'path';
+import fs from 'node:fs';
+import console from 'console';

examples/flat/src/jsx.tsx

@@ -0,0 +1,3 @@
+const Components = () => {
+  return <></>;
+};

examples/flat/tsconfig.json

@@ -0,0 +1,14 @@
+{
+  "compilerOptions": {
+    "jsx": "react-jsx",
+    "lib": ["ESNext"],
+    "target": "ESNext",
+    "module": "ESNext",
+    "rootDir": "./",
+    "moduleResolution": "Bundler",
+    "esModuleInterop": true,
+    "forceConsistentCasingInFileNames": true,
+    "strict": true,
+    "skipLibCheck": true
+  }
+}