Create a Shared Project for `Rocks.Analysis` and `Rocks.Completions`

[JasonBock]

Describe the solution you'd like
In AddRockAttributeRefactoring, there's a section of code where I could check the target type to mock and only offer the refactoring if it could be mocked. But this would mean taking a reference on Rocks.Analysis, which...probably don't want to do that. An arguably better approach is to create a shared project - call it Rocks.Compilation - that both could reference.

There may be a need for this between Rocks.Analysis and Rocks as well - e.g. BuildType is the same. Or, what I could have is a shared folder that contains the code and that way it's the "same" between them. Just depends on how much NuGet wrangling is needed in the package.