Migrate ESLint 8 → 9

- Migrates to flat config
- Replaces eslint-plugin-import with eslint-plugin-import-x

eslint.config.js

@@ -0,0 +1,59 @@
+/* eslint-disable import-x/no-extraneous-dependencies */
+import js from '@eslint/js';
+import eslintPluginImportX from 'eslint-plugin-import-x';
+import globals from 'globals';
+import { configs as tsConfigs, parser as tsParser } from 'typescript-eslint';
+
+export default [
+  js.configs.recommended,
+  eslintPluginImportX.flatConfigs.recommended,
+  eslintPluginImportX.flatConfigs.typescript,
+  ...tsConfigs.recommended,
+  {
+    languageOptions: {
+      parser: tsParser,
+      globals: {
+        ...globals.node,
+      },
+      ecmaVersion: 2020,
+      sourceType: 'module',
+    },
+    rules: {
+      'import-x/first': 'error',
+      'import-x/exports-last': 'error',
+      'import-x/newline-after-import': 'error',
+      'import-x/prefer-default-export': 'error',
+      'import-x/group-exports': 'error',
+      'import-x/no-duplicates': 'error',
+      'import-x/no-amd': 'error',
+      'import-x/no-commonjs': 'error',
+      'import-x/order': [
+        'error',
+        {
+          groups: [
+            'builtin',
+            'external',
+            'internal',
+            'parent',
+            'sibling',
+            'index',
+          ],
+          'newlines-between': 'always',
+
+          alphabetize: {
+            order: 'asc',
+          },
+        },
+      ],
+      'import-x/no-unused-modules': 'error',
+      'import-x/no-mutable-exports': 'error',
+      'import-x/no-extraneous-dependencies': [
+        'error',
+        {
+          devDependencies: ['src/**/*.test.ts'],
+        },
+      ],
+      '@typescript-eslint/explicit-function-return-type': 'error',
+    },
+  },
+];

package.json

@@ -35,13 +35,14 @@
     "prepack": "npm run build"
   },
   "devDependencies": {
+    "@eslint/js": "^9.11.0",
     "@types/bun": "^1.1.8",
-    "@typescript-eslint/eslint-plugin": "^8.4.0",
-    "eslint": "^8.57.0",
+    "eslint": "^9.11.0",
     "eslint-import-resolver-typescript": "^3.6.3",
-    "eslint-plugin-import": "^2.30.0",
+    "eslint-plugin-import-x": "^4.2.1",
     "prettier": "^3.3.3",
     "typescript": "5.4.3",
+    "typescript-eslint": "^8.6.0",
     "vitest": "^2.0.5"
   }
 }