feat: Hover and Autocomplete for Gas directives

[WillLillis]

This PR (sorry it's a bit long!) adds hover and autocomplete support for Gas directives. Adding this was relatively straightforward code-wise, as the majority of time went into writing the XML file. I initially looked into writing a python script to pull the information together, but there were enough oddities/ things to manually edit in the docs that I just wrote it by hand (The LuaSnip plugin was very helpful in this regard).

Huge thanks to @Freed-Wu for making the request and linking the relevant documentation!

Closes #46