feat: plpgsql check

[psteinroe]

adds support for plpgsql_check. the approach is even simpler as described in the issue: if we encounter a create function statement, we start a transaction, run the statement, then run plpgsql_check on it, and then rollback the transaction.

the remaining code is just about translating the location information we get from the extension into a good range.

todo:

• integrate into workspace api
• to get span, move from the first word / occurrence in the line to the next semicolon
• handle return trigger by checking for all variations
• make sure we do not report create fn syntax errors if plpgsql_check is enabled
• check why "not a known variable" is not getting the right span

closes #190