Add ThisAssembly.Git

[kzu]

This is something that GitInfo provides, but it does so using MSBuild and it seems to cause an issue in some source-generator scenarios.

So perhaps it would be best if the codegen part was provided by a source generator in this project instead, with GitInfo detecting its presence (via a ThisAssemblyGitInfo=true property perhaps?) and avoid generating the ThisAssembly.Git part of the code.

It might even make sense for GitInfo to take a dependency on the new ThisAssembly.Git entirely and just augment it for the semver determination.

This implementation should likely rely on SourceLink implementation for some information retrieval, and take a dependency on it to avoid duplicating work?