MessageSourceLoader class abstract

Interface for loading messages from different resource formats.

This abstraction allows the MessageSource system to support multiple file formats (JSON, YAML, Properties) through pluggable loaders. Each loader is responsible for parsing its specific format and returning a flat map of message keys to values.

Example implementation:

class JsonMessageLoader implements MessageSourceLoader {
  @override
  Future<Map<String, String>> load(AssetPathResource resource) async {
    final parser = JsonParser();
    final data = parser.parse(resource.getContentAsString());
    return _flattenMap(data);
  }
}