Make Visual Studio provide `FSharpSource` objects based on live buffers

Following up on https://github.com/dotnet/fsharp/issues/11976

It should also fix https://github.com/dotnet/fsharp/issues/5915

Prototype PR: https://github.com/dotnet/fsharp/pull/11588