integrate plpgsql_check

[psteinroe]

since we have a db connection, we should be able to integrate plpgsql_check. I would propose two layers:

1. inline lints on the source code itself: actively query the extension for the function we detect in the source and show lints. we might even be able to show the exact location of the lints within the function body.
2. list issues in the database unrelated to the source code: show all results for all functions. these lints can get quite long quite fast, so we probably want to allow the user to filter out certain schemas / functions / issues types / error levels.

I would first go with 1), and then try to figure out a good way to do 2).