Add tab-complete for ALTER DOMAIN ADD [CONSTRAINT]

Add tab-complete for ALTER DOMAIN ADD  [CONSTRAINT]

We can add tab-completion with "CHECK (" and "NOT NULL" after ALTER
DOMAIN ADD [CONSTRAINT].

ALTER DOMAIN dom ADD -> CHECK (
ALTER DOMAIN dom ADD -> NOT NULL
ALTER DOMAIN dom ADD -> CONSTRAINT
ALTER DOMAIN dom ADD CONSTRAINT nm -> CHECK (
ALTER DOMAIN dom ADD CONSTRAINT nm -> NOT NULL

Author: jian he <jian.universality@gmail.com>
Author: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Discussion: https://postgr.es/m/CACJufxG_f6LzAT_McC-kKmQWpuWnOYKyNBw8Kv3xzTjPqmeHcA@mail.gmail.com