Using the Build Server

The existing build system (`godot mono` or the alternative `godot-dotnet`) uses `progress` to launch DotNet CLI to perform build tasks.

In most cases, this works well. But for situations where you need to execute build tasks frequently (I think this is the case when developing games or editor extensions in C#), there is some overhead in cold starting the build task every time.

Rolsyn provides a new mode ([Compiler Server](https://github.com/dotnet/roslyn/blob/main/docs/compilers/Compiler%20Server.md)) to alleviate this overhead.

I'm glad to see that we can switch our build system to `compiler server` mode!

Start by reading the existing `progress` invocation used by the `godot mono` and `godot-dotnet` build systems, then review Roslyn's Compiler Server documentation linked in the issue. Determine how compiler-server mode should be enabled for these build tasks and verify that repeated builds avoid the current cold-start overhead.

Written by the indexing model from the issue text.