Implement Language Server and Client for ShEx in javascript to support IDE's

[thadguidry]

Having a Language Server & Client written in Javascript means that many IDE's can provide support for ShEx language beyond simple syntax highlighting by additionally providing language references as they do with other languages.
Example: https://code.visualstudio.com/api/language-extensions/overview

[ericprud]

I built a ShExC mode for ace to address this. I overhauled the shex.js parsing error convention so the lint worker thread can render multiple issues. The mode still needs more work but shows what can be done.

IIRC, @labra built a ShExC mode for codemirror.