Allow for a way to ensure some source generators run before / after others

[pranavkm]

We briefly discussed this during 6.0 / C# 10 but ultimately punted it since the ask came in too late. We've had issues where user-written source generators can no longer observe the output of source generated by Razor's:

• Razor Components which are added by a 3rd party source generator do not render if they are added to a razor file using markup syntax. aspnetcore#32253
• Unable to use Blazor components in source generators aspnetcore#32949
• Components added by a source generator work in code, but not in markup aspnetcore#32172

For 6, the solution we gave our users is to turn off the source generator and fall back to our 2-phase compilation process. We'd made some progress discussing this internally, but filing this for C# 11 consideration.

[bugproof]

I really want to use this feature with Blazor but seems like I have to rely on AOP frameworks instead.

[jcouv]

Another instance of this problem which was reported is using RegexGenerator in Blazor project.

[MarkFl12]

Yeah, I've been trying to build a source generator to help with Blazor projects, but I think what's happening is that it needs to run before the generator that transforms the .razor files so that when their generator runs the code I've generated that's used in them is available