Add plpgsql_check extension

[kiwicopple]

There is significant demand for the plpgsql_check extension:
https://github.com/supabase/postgres/issues/5#issuecomment-636765436

plpgsql_check is for linting and validating SQL code quality. It looks like it is well maintained and is supported on PG12.