Add basic `msbuild` support for C# projects

This MR adds very basic support for C# projects based on .csproj msbuild files.

I've copied most of the build/maven.go logic into a new build/msbuild.go file, and registered MSBuild as a compatible build tool. I've also stubbed out all of the dependency methods as we don't need them yet, and added a minimal C# project as a test case.