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/build_package.sh

@@ -27,7 +27,7 @@ fi
 tsc --build
 
 # copy .{md,json} files
-node ../../scripts/copy_files.cjs
+node ../../scripts/copy_files.js
 
 # touch dist/.last_build
 touch dist/.last_build

scripts/copy_files.js

@@ -1,4 +1,11 @@
 #!/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");
@@ -13,8 +20,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-const fs = require('fs');
-const path = require('path');
+import fs from 'fs';
+import path from 'path';
 
 const sourceDir = path.join('src');
 const targetDir = path.join('dist', 'src');