Create an implementation of the LSP for X#

[jp2masa]

Note (28/11/2017): VS support for LSP seems to be mainly focused on open folder scenarios, so we should wait for now.

We should implement the LSP (Language Server Protocol) for X#, which will work for many IDEs, including VS Code and VS (right now it's a preview extension).
Tools supporting the LSP: https://microsoft.github.io/language-server-protocol/implementors/tools/
There are currently 2 C# implementations of the protocol, and an official one is under development: https://microsoft.github.io/language-server-protocol/implementors/sdks/