support plpgsql function bodies

[psteinroe]

once we have #178, we can start looking into this too. pg_query.rs already exposes a basic binding for parsing plpgsql. we should first improve that binding upstream and then start adding language features like autocompletion.

[mpayette-3rdeyecam]

Very much looking forward to this. Given that most of our non-trivial sql code is functions rather than regular schema tables and whatnot, not having the language server parse those means we lose a lot of error checking.