Build: Add .yarn/ and package-lock.json to .gitignore

- Ignore .yarn/ directory (yarn cache and plugins)
- Ignore package-lock.json (npm lock file)
- These are build artifacts that shouldn't be committed

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: ianhi

.gitignore

@@ -20,6 +20,8 @@ lib/
 ipympl/nbextension/index.*
 ipympl/nbextension/package.json
 ipympl/labextension/*.tgz
+.yarn/
+package-lock.json
 
 ui-tests/test-results
 ui-tests/playwright-report