Dual packaging, without breaking existing consumers (#628)

Co-authored-by: Yasumasa Ashida <ys.ashida@gmail.com>

eslint.config.mjs

@@ -0,0 +1,36 @@
+import js from '@eslint/js';
+import importPlugin from 'eslint-plugin-import';
+import { configs, plugins } from 'eslint-config-airbnb-extended';
+
+export default [
+  js.configs.recommended,
+  importPlugin.flatConfigs.recommended,
+  // Stylistic plugin
+  plugins.stylistic,
+  // Import X plugin
+  plugins.importX,
+  // Airbnb base recommended config
+  ...configs.base.recommended,
+  {
+    files: ['**/*.{js,mjs,cjs}'],
+    languageOptions: {
+      ecmaVersion: 'latest',
+      sourceType: 'module',
+    },
+    rules: {
+      'no-underscore-dangle': 0,
+      'no-void': 0,
+      'import/prefer-default-export': 1,
+      'import-x/extensions': [
+        'error',
+        'always',
+        {
+          ignorePackages: true,
+          js: 'always',
+        },
+      ],
+      '@stylistic/max-len': 0,
+      '@stylistic/arrow-parens': 0,
+    },
+  },
+];