Refactor: Convert copy_files.cjs to ES module syntax

- Converted scripts/copy_files.cjs to use ES module syntax (renaming to copy_files.js).
- This change aligns with the project's preference for ES modules over CommonJS for better modernity and future-proofing.
- Updated eslint.config.js to remove the .cjs override.
- Adjusted scripts/build_package.sh to call the new .js file.

scripts/copy_files.js

@@ -0,0 +1,56 @@
+#!/usr/bin/env node
+
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import fs from 'fs';
+import path from 'path';
+
+const sourceDir = path.join('src');
+const targetDir = path.join('dist', 'src');
+
+const extensionsToCopy = ['.md', '.json', '.sb'];
+
+function copyFilesRecursive(source, target) {
+  if (!fs.existsSync(target)) {
+    fs.mkdirSync(target, { recursive: true });
+  }
+
+  const items = fs.readdirSync(source, { withFileTypes: true });
+
+  for (const item of items) {
+    const sourcePath = path.join(source, item.name);
+    const targetPath = path.join(target, item.name);
+
+    if (item.isDirectory()) {
+      copyFilesRecursive(sourcePath, targetPath);
+    } else if (extensionsToCopy.includes(path.extname(item.name))) {
+      fs.copyFileSync(sourcePath, targetPath);
+    }
+  }
+}
+
+if (!fs.existsSync(sourceDir)) {
+  console.error(`Source directory ${sourceDir} not found.`);
+  process.exit(1);
+}
+
+copyFilesRecursive(sourceDir, targetDir);
+console.log('Successfully copied files.');