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.

src/test/regress/sql/plpgsql.sql

@@ -3189,3 +3189,18 @@ $$ language plpgsql;
 select * from conflict_test();
 
 drop function conflict_test();
+
+-- Check that an unreserved keyword can be used as a variable name
+
+create function unreserved_test() returns int as $$
+declare
+  forward int := 21;
+begin
+  forward := forward * 2;
+  return forward;
+end
+$$ language plpgsql;
+
+select unreserved_test();
+
+drop function unreserved_test();