[all pacakges] Adopt .g.dart for all generated files #3785

When reviewing PRs it doesn't show the first file of the PR necessarily. If the filename itself doesn't contain an indication the file is generated, you might accidentally review generated files.

Also we use _generated in some places which is overly verbose.

@liamappelbe what do you think of adopting .g.dart for generated Dart files everywhere in this repo?