Update solution file
Update solution file

This solution file has been created with the `dotnet sln add` command, and thus is more consistent with what people would expect

robkeim/xcsharp,ErikSchierboom/xcsharp,exercism/xcsharp,robkeim/xcsharp,GKotfis/csharp,exercism/xcsharp,GKotfis/csharp,ErikSchierboom/xcsharp