Fix plpgsql to allow new-style SQL CREATE FUNCTION as a SQL command.

plpgsql fails on new-style CREATE FUNCTION/PROCEDURE commands within
a routine or DO block, because make_execsql_stmt believes that a
semicolon token always terminates a SQL command.  Now, that's actually
been wrong since the day it was written, because CREATE RULE has long
allowed multiple rule actions separated by semicolons.  But there are
few enough people using multi-action rules that there was never an
attempt to fix it.  New-style SQL functions, though, are popular.

psql has this same problem of "does this semicolon really terminate
the command?".  It deals with CREATE RULE by counting parenthesis
nesting depth: a semicolon within parens doesn't end a command.
Commits e717a9a18 and 029c5ac03 created a similar heuristic to count
matching BEGIN/END pairs (but only within CREATEs, so as not to be
fooled by plain BEGIN).  That's survived several releases now without
trouble reports, so let's just absorb those heuristics into plpgsql.

Per report from Samuel Dussault.  Back-patch to v14 where new-style
SQL function syntax came in.

Discussion: https://postgr.es/m/YT2PR01MB88552C3E9AD40A6C038774A781722@YT2PR01MB8855.CANPRD01.PROD.OUTLOOK.COM

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