Out-of-process host for MSBuild

[tintoy]

At present, MSBuild has to be loaded into the same process as the language server, which can be problematic when the required version of MSBuild does not target the same SDK / runtime as the language server.

High-level design:

[VSCode Extension] ==LSP==> [Language Server] ==MIP==> [MSBuild Server]

• LSP = Language Server Protocol
• MIP = MSBuild Inspector Protocol)
• MSBuild Server = process with some sort of API endpoint, hosting an MSBuild ProjectCollection

LSP is basically JSON-RPC, but has requirements which have resulted in it being designed that way.
For MIP, we can probably get away with GRPC or similar.

Tricky bits:

• Selecting correct SDK and runtime for MSBuild Server
• Launching MSBuild Server so that it uses the correct SDK and runtime
• Managing project state, without making the API too chatty