ConfigurableMessageSource constructor

Main implementation of MessageSource that supports loading messages from multiple resources and formats.

This class allows you to configure message sources by loading different message files for different locales using pluggable loaders for various formats (JSON, YAML, Properties).

Example usage:

final messageSource = ConfigurableMessageSource(
  defaultLocale: Locale('en', 'US')
);

// Load English messages from JSON
await messageSource.loadMessages(
  Locale('en'),
  AssetPathResource('messages_en.json'),
  loader: JsonMessageLoader(),
);

// Load French messages from YAML  
await messageSource.loadMessages(
  Locale('fr'),
  AssetPathResource('messages_fr.yaml'),
  loader: YamlMessageLoader(),
);

// Use the messages
print(messageSource.getMessage('greeting', locale: Locale('fr')));