use langrefs keyword reference to provide hover for keywords

[xdBronch]

https://ziglang.org/documentation/master/#Keyword-Reference
same idea as how zls currently uses the langref for docs on builtin functions
example from rust-analyzer

[Techatrix]

Related issue #1174

I though about using the keyword reference but there were some TODOs and descriptions that didn't add much value:

struct defines a struct.

WHAT?!? struct defines a struct! Who would have though?

[xdBronch]

lol yeah thats fair but i think a few have at least some value. hopefull theyll be expanded on later