add framework for LSP support

[brettfo]

This is woefully and purposefully lacking in functionality as it's meant to lay the groundwork for an eventual move to an official Language Server Protocol implementation.

While there are other F# LSP projects out there, this one is being designed slightly differently, namely with one assembly I hope to use FSharp.Compiler.LanguageServer as:

• a regular IL reference for consumption from other F# libraries (also from C#/VB, but the async interop is odd)
• a regular IL reference for LSP over System.IO.Stream objects
• a console application that communicates via stdin/stdout
• the important one: strongly tied to a specific version of the compiler and editor tools

For now this project exists in isolation, but eventually FSharp.Editor will use these APIs and basically be a small shim between FSharp.Compiler.LanguageServer and Visual Studio.

[KevinRansom]

For anyone interested in LSP here is a link: https://en.wikipedia.org/wiki/Language_Server_Protocol