Grammatical Framework language support for Visual Studio Code

Adds syntax highlighting to Grammatical Framework (.gf) files for Visual Studio Code.

The extension is based on:

• John Camilleri's extension for Atom
• Rob Clouth's work porting it to VS Code
• Peter Ljunglöf's improvements
• Andreas Källberg's language server client extension

To learn more about GF visit: https://www.grammaticalframework.org