AbstractMessageSource constructor

Base implementation of MessageSource providing common functionality.

This abstract class implements shared behavior like placeholder substitution and fallback locale resolution, allowing concrete implementations to focus on message loading and storage.

Key features:

• Placeholder substitution with {0}, {1}, etc.
• Locale fallback chain resolution
• Message formatting utilities

Example subclass:

class MyMessageSource extends AbstractMessageSource {
  @override
  String? resolveMessage(String code, Locale locale) {
    // Custom message resolution logic
    return myMessages[locale]?[code];
  }
}