fix: prepend eslint default configs (#66)

Co-authored-by: Anthony Fu <github@antfu.me>

src/configs.ts

@@ -104,6 +104,41 @@ export async function readConfig(
   if (!Array.isArray(rawConfigs))
     rawConfigs = [rawConfigs]
 
+  // ESLint applies these default configs to all files
+  // https://github.com/eslint/eslint/blob/21d3766c3f4efd981d3cc294c2c82c8014815e6e/lib/config/default-config.js#L66-L69
+  rawConfigs.unshift(
+    {
+      name: 'eslint/defaults/languages',
+      languageOptions: {
+        sourceType: 'module',
+        ecmaVersion: 'latest',
+        parserOptions: {},
+      },
+      linterOptions: {
+        reportUnusedDisableDirectives: 1,
+      },
+    } as FlatConfigItem,
+    {
+      name: 'eslint/defaults/ignores',
+      ignores: [
+        '**/node_modules/',
+        '.git/',
+      ],
+    } as FlatConfigItem,
+    {
+      name: 'eslint/defaults/files',
+      files: ['**/*.js', '**/*.mjs'],
+    } as FlatConfigItem,
+    {
+      name: 'eslint/defaults/files-cjs',
+      files: ['**/*.cjs'],
+      languageOptions: {
+        sourceType: 'commonjs',
+        ecmaVersion: 'latest',
+      },
+    } as FlatConfigItem,
+  )
+
   const rulesMap = new Map<string, RuleInfo>()
 
   // Try resolve `eslint` module from the same directory as the config file