proposal - reference to plpgsql_check from plpgsql doc

Hi

plpgsql_check extension is almost complete now. This extension is available on all environments and for all supported Postgres releases. It is probably too big to be part of contrib, but I think so it can be referenced in https://www.postgresql.org/docs/current/plpgsql-development-tips.html chapter.

What do you think about it?

Regards

Pavel