Add eXist-db syntax parser and implement pre-commit hook config

[mcmeeking]

Implements the parser from #48 with some minor modifications to support unary lookups (? unclear on the correct terminology, I'm referring to this syntax for map lookups:

let $something := $ns:variable?key

) and to bring that branch up to date with master.

Also introduces pre-commit hook config for xqlint, so pre-commit can be pointed to this repo at a given revision and xqlint will be run on any files matching \.xq(l|m)?$.

I've tested this on xquery files with exist-db syntax, and have run grunt but have not added any additional tests as my JS is not strong at all.

Closes:

• update insert #57
• XQuery 3.1 Map Syntax #77