fix: flutterfire configure command when existing firebase_options.dart has a wrong format

[nilsreichardt]

Description

If the repository already had a firebase_options.dart file, the flutterfire configure commend expected a specific format. If this file had a different format, the flutterfire configure . command failed.

As described in #422 the example of flutter/ai repository has this firebase_options.dart:

// File normally generated by FlutterFire CLI. This is a stand-in.
// See README.md for details.
import 'package:firebase_core/firebase_core.dart' show FirebaseOptions;

class DefaultFirebaseOptions {
  // TODO: Remove this file and run `flutterfire config` for Firebase options
  static FirebaseOptions get currentPlatform {
    throw UnimplementedError(
      'Generate this file by running `flutterfire configure`. '
      'See README.md for details.',
    );
  }
}

Source: https://github.com/flutter/ai/blob/fd3d6d7936f503e9e53d91564cb280263083c1bf/example/lib/firebase_options.dart

Before this PR, the flutterfire configure command couldn't handle this firebase_options.dart file.

Closes #422

Type of Change

• ✨ feat -- New feature (non-breaking change which adds functionality)
• 🛠️ fix -- Bug fix (non-breaking change which fixes an issue)
• ❌ ! -- Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 refactor -- Code refactor
• ✅ ci -- Build configuration change
• 📝 docs -- Documentation
• 🗑️ chore -- Chore