Better manifestation of analyzer diagnostics

[AArnott]

We should move more or all of our diagnostics from the source generator to their own analyzers. Analyzers run more frequently by default than source generators in VS, and thus users are far more likely to see the warnings/errors that they emit. Source generated warnings may only appear in full builds at least for some user configurations.

Following is a discussion where a user was unable to discover that partial needed to be added to their declarations because the warning never showed up in the VS error list (though it did appear in the full build log):
AArnott/Nerdbank.MessagePack#466 (comment)

[AArnott]

Another downside of source generator-emitted diagnostics (apparently) is that they cannot be suppressed with #pragma warning disable around the location.

[eiriktsarpalis]

Yep, we have a tracking issue for that: dotnet/runtime#92509 (comment)