Add TLA+ language server extension

[pmer]

We can add support for these features in TLA+ code:

• Go To Definition for constants
• Errors / warnings (from SANY)

within VS Code, Eclipse Che, Eclipse, Emacs, GNOME builder, and Atom. And any more IDEs that adopt the language server protocol in the future.

Resources

• Blog post where it was introduced
• Blog post where Eclipse Che announces support
• Tutorial to make our own language server (with VS Code)