fix: migrate eslint example to flat config

Author: chenjiahan

rspack/eslint/.eslintrc.js

@@ -0,0 +1,12 @@
+export default [
+  {
+    files: ['src/**/*.js'],
+    languageOptions: {
+      ecmaVersion: 'latest',
+      sourceType: 'module',
+      globals: {
+        document: 'readonly',
+      },
+    },
+  },
+];