VS: Add support for per-config C# sources
Note: I'm not sure how to test a condition where multiple configurations are accepted (MinSizeRel and Release pointing to the same file, for example, whereas Debug points to another one). It does seem to work with the minimal example linked to from #22108 (closed).
Commands I tested with:
cmake.exe -G "Visual Studio 15 2017" -A x64 -S D:\CMakeCSharpRepro -B D:\CMakeCSharpReproB cmake.exe -G "Visual Studio 16 2019" -A x64 -S D:\CMakeCSharpRepro -B D:\CMakeCSharpReproB
Fixes: #22108 (closed)