Parser-based completion and assistance

[esdmr]

Currently, for completion, we send the whole script up to the cursor to fish, and for assistant we use a rather horrible RegExp to parse stuff, which has a ton of false negatives (and hopefully no false positives).

What if we were to create a parser for fish shell and use that to find functions and variables and only send the current command to fish to complete? It also fixes any problem with finding the selected argument and if and while statements, which are broken for some reason.

Edit: See https://github.com/esdmr/fish-parsing-parables