Automatic AdditionalFiles and source generator reference in non-Razor SDK projects

[davidwengier]

Cohosting requires the Razor source generator be referenced in a project, and the source generator requires that documents be AdditionalFiles. When these things don't happen, either because someone is not using the Razor SDK, or they have just customized their build in some way (eg, MAUI Blazor used to do this, maybe still does) we need to override things on the Roslyn side, to ensure tooling will work.