Move AST to separate package

[riz0id]

This PR isolates Proto3.Suite.DotProto.AST into separate package proto3-suite-ast.

[Gabriella439]

The original request for this was from quite a while ago so I wanted to check if we still need to split this out into a separate package?

I do think there is a good reason to split into multiple packages, but for a different reason: I think there should be a package that implements the code generation logic and the generated code should depend on a separate package. However, I don't believe separating out the AST into a separate package addresses that.

[riz0id]

I can't think of a reason why the syntax datatype would be useful in a separate package even if I went ahead and added the parser into the new package. I don't mind removing the new package, but I do think that some of the changes in this PR like documentation, removing orphan instances out of Proto3.Suite.DotProto.Rendering, and the new modules compartmentalizing things a bit more are useful though.