Source generators should annotate generated types/members with [GeneratedCode]

[stevemonaco]

Is your feature request related to a problem? Please describe.

There is a use case in HotAvalonia where fetching fields associated with the AvaloniaNameSourceGenerator would have been more straightforward. Instead, an approach using bytecode analysis was necessary.

Describe the solution you'd like

Types/members generated by code should be annotated with [GeneratedCode], which is common convention. Definitely for AvaloniaNameSourceGenerator, but maybe also for the various DevGenerators?

Describe alternatives you've considered

No response

Additional context

No response