addToGitignore function
Appends entries to the project's .gitignore (created if missing).
Implementation
void addToGitignore(FlutterProject project, List<String> entries) {
final f = File(p.join(project.dir, '.gitignore'));
final existing = f.existsSync() ? f.readAsStringSync() : '';
final sep = existing.isEmpty || existing.endsWith('\n') ? '' : '\n';
f.writeAsStringSync(
'$existing$sep\n# flutter_buildkit\n${entries.join('\n')}\n');
}