Add C# benchmarks from exercism/csharp

[waf]

Copies in the benchmarks from exercism/csharp repository (from the exercises/practice directory). This is just a direct copy of that repository's subdirectory.

I think I've set up this csharp directory structure correctly, following the pattern of other languages, but if I've missed something I'm happy to make updates.

If this PR is accepted, I plan to follow it up with a pull request to the Aider AI benchmarking application (https://github.com/Aider-AI/aider/tree/main/benchmark). For that future pull request, the latest dotnet is in the ubuntu repositories so I think any Dockerfile modifications will be minimal, and most of the work will focus on the python program to call the dotnet command line.