[VSCode] Switch to single server

[allisonchou]

Our VSCode handlers should eventually switch over to single server like in VS.

[davidwengier]

I updated the code to keep track of where we are with this. The list is here: https://github.com/dotnet/razor/blob/main/src/Razor/src/Microsoft.AspNetCore.Razor.LanguageServer.Common/RazorLanguageServerCustomMessageTargets.cs#L29-L42