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

scripts/build_package.sh

@@ -26,8 +26,8 @@ fi
 # build typescript files
 tsc --build
 
-# copy .{md,json} files  (replace -q with -i to see itemized changes)
-rsync -aq --delete --include='*.md' --include='*.json' --include='*.sb' --include='*/' --exclude='*' ./src/ ./dist/src/
+# copy .{md,json} files
+node ../../scripts/copy_files.cjs
 
 # touch dist/.last_build
 touch dist/.last_build