This adds tab completion of the clauses WHEN and EXECUTE
FUNCTION|PROCEDURE clauses to CREATE EVENT TRIGGER, similar to CREATE
TRIGGER in the previous commit.  This has version-dependent logic so as
FUNCTION is chosen over PROCEDURE for 11 and newer versions.

Author: Dagfinn Ilmari Mannsåker
Reviewed-by: Tom Lane, Michael Paquier
Discussion: https://postgr.es/m/d8jmur4q4yc.fsf@dalvik.ping.uio.no