Remove plpgsql's separate lexer (finally!), in favor of using the core lexer

Remove plpgsql's separate lexer (finally!), in favor of using the core lexer
directly.  This was a lot of trouble, but should be worth it in terms of
not having to keep the plpgsql lexer in step with core anymore.  In addition
the handling of keywords is significantly better-structured, allowing us to
de-reserve a number of words that plpgsql formerly treated as reserved.