feat(completions): ts_query package, column autocompletion

[juleswritescode]

This PR does two things:

1. It adds autocompletion for Columns
2. It adds a pg_treesitter_queries crate which cares about extracting useful information from a tree-sitter tree (such as "which tables are mentioned?")

In sum, that makes it such that we have smart autocompletion for columns – see the test cases in pg_completions/src/complete.rs for a sneakpeek :)

I chose to put thepg_treesitter_queries into their own crate so we can reuse them for pg_hover and the likes.

In addition, I created an InputQuery struct for our test_helpers which enforces that callers put a CURSOR_POS into their queries – I had a nasty bug before because I forgot to do it.

[psteinroe]

really nice and clean, love it!! answered your questions and left few comments but overall lgtm!