[O# Parity] Project loading support for projects running on Mono

[dibarbet]

For the C# project loader, we need to run our design time builds out of proc using the project specific SDK. This will avoid errors trying to load various project types in the .NET 7 runtime.

With out of proc design time builds, we should also be able to load mono / legacy csproj (similar to O#'s useModernDotnet flag).

Note that this is only planned to be supported in the C# extension project loader.