Add language server package

[d-torrance]

This is a (very very very) early draft of a LanguageServer package that implements Microsoft's Language Server Protocol (LSP), which is used by multiple editors (Emacs, VSCode, etc.)

[mahrud]

Is there a TODO list for this? Is the idea that LSP should fully replace the M2-mode on Emacs and extensions on Vim and VSCode? If so it would be good to have a list of M2-mode features that need to be reimplemented.

[d-torrance]

A TODO list would be good. For the time being, I was just focusing on getting the basic lifecycle messages working and hadn't put much thought into what actual features to implement. So current TODO list:

• Get life cycle messages working
• Write a TODO list :)

I don't think LSP would replace M2-mode in Emacs, just supplement it. That seems to be the case for the other major modes for languages that already have LSP support, like python-mode.

I'm also thinking of breaking the JSON-RPC stuff, which is essentially fully functional, into its own package (or maybe just add to the existing JSON package). It's not unique to LSP, and potentially might be useful for some other stuff. It's apparently widely used in cryptocurrency applications, for example.