Add tab completion of EXECUTE FUNCTION for CREATE TRIGGER in psql

Add tab completion of EXECUTE FUNCTION for CREATE TRIGGER in psql

The change to accept EXECUTE FUNCTION as well as EXECUTE PROCEDURE in
CREATE TRIGGER (added by 0a63f99) forgot to tell psql's tab completion
system about this.  In passing, add tab completion of EXECUTE
FUNCTION/PROCEDURE after a complete WHEN ( … ) clause.

This change is version-aware, with FUNCTION being selected automatically
instead of PROCEDURE depending on the backend version, PROCEDURE being
an historical grammar kept for compatibility and considered as
deprecated in v11.

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