Add support for extensions to SyntaxGenerator APIs

[jcouv]

This is a follow-up from discussion with Eric who mentioned that the GenAPI tool depends on our SyntaxGenerator tool and would need this.
@ericstj Could you add any appropriate details?

[ericstj]

GenAPI is used to produce source-build-reference-packages as stand-in source for packages when we cannot consume binaries.

These need to be able to represent all the public surface of an assembly. We also use the same codebase to produce API diffs like https://github.com/dotnet/core/tree/main/release-notes/10.0/preview/preview3/api-diff.

We need to describe how SyntaxGenerator handles new extensions. This might also be needed for Object Browser and go-to-definition.

[CyrusNajmabadi]

I can do this work