Fix: Use Node.js script for cross-platform file copying in build

- Replaces the rsync command in the build_package.sh script with a
  Node.js script (copy_files.cjs) to handle copying necessary files
  (e.g., .md, .json, .sb) during the build process.
- Addresses an issue where the build would fail on systems
  that do not have rsync installed (e.g., some Windows environments or
  minimal Linux distributions) by using a Node.js script, providing a
  cross-platform solution as Node.js is already a project dependency.
- Updates the ESLint configuration to correctly lint .cjs files as
  CommonJS modules.

Fixes https://github.com/google-gemini/gemini-cli/issues/387

eslint.config.js

@@ -154,6 +154,34 @@ export default tseslint.config(
       },
     },
   },
+  // Override for .cjs files to use CommonJS
+  {
+    files: ['**/*.cjs'],
+    languageOptions: {
+      sourceType: 'commonjs',
+      globals: {
+        ...globals.node, // Add all Node.js globals
+        __dirname: 'readonly',
+        __filename: 'readonly',
+        exports: 'writable',
+        module: 'readonly',
+        require: 'readonly',
+      },
+    },
+    rules: {
+      // Disable rules that are not applicable to CommonJS
+      '@typescript-eslint/no-require-imports': 'off',
+      'no-restricted-syntax': [
+        'error',
+        // Keep other restricted syntaxes, but allow require for .cjs
+        {
+          selector: 'ThrowStatement > Literal:not([value=/^\\\\w+Error:/])',
+          message:
+            'Do not throw string literals or non-Error objects. Throw new Error("...") instead.',
+        },
+      ],
+    },
+  },
   // Prettier config must be last
   prettierConfig,
   // Custom eslint rules for this repo