Annotate `MonarchTheme` on async getter

[KestasVenslauskas]

This feature would help some initialization when using additional tools to generate theme.

[fertrig]

Would something like this work for you?

import 'package:monarch_annotations/monarch_annotations.dart';

@AsyncMonarchTheme('Fancy Light Theme')
Future<ThemeData> get fancyLightTheme async {
  var theme = await someFutureThatReturnsThemeData();
  return theme;
}

[KestasVenslauskas]

@fertrig Yes, that would help, because sometimes we have to init theme with other tools that has to be initialised beforehand.