[PATCH] Tab complete EXECUTE FUNCTION for CREATE (EVENT) TRIGGER

Hi hackers,

The last-minute change for CREATE (EVENT) TRIGGER to accept EXECUTE
FUNCTION as well as EXECUTE PROCEDURE did not update the tab completion
in psql to match.  Please find attached two patches that do this for
CREATE TRIGGER and CREATE EVENT TRIGGER, respectively.

To keep the duplication minimal, I've changed it from completing EXECUTE
PROCEDURE as a single thing to just EXECUTE, and then completing
FUNCTION or FUNCTION and PROCEDURE after that depending on the server
version.

- ilmari
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