refactor: update localization handling to fall back to English and im…

[ragokan]

Description

Localization fallback improvements:

• The loc getter in the LocalizationsExt extension now falls back to the default English localization (FlutterQuillLocalizationsEn) if no delegate is registered, instead of throwing a MissingFlutterQuillLocalizationException.
• The MissingFlutterQuillLocalizationException class is now marked as deprecated, as it will be removed in a future release.

Test updates:

• Tests in editor_test.dart were updated to verify that no exception is thrown when the localization delegate is missing, and that English fallback messages are used. [1] [2]
• Tests now check that the provided localization delegate is used when available (e.g., Spanish), and that fallback localization provides correct interpolated messages when the delegate is missing.

Related Issues

• Related Fallback to en_US when FlutterQuillLocalizations.delegate is not provided #2609 Fallback to en_US when FlutterQuillLocalizations.delegate is not provided #2609

Type of Change

• ✨ Feature: New functionality without breaking existing features.
• 🛠️ Bug fix: Resolves an issue without altering current behavior.
• 🧹 Refactor: Code reorganization, no behavior change.
• ❌ Breaking: Alters existing functionality and requires updates.
• 🧪 Tests: New or modified tests
• 📝 Documentation: Updates or additions to documentation.
• 🗑️ Chore: Routine tasks, or maintenance.
• ✅ Build configuration change: Build/configuration changes.