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

src/bin/psql/tab-complete.in.c

@@ -2539,6 +2539,12 @@ match_previous_words(int pattern_id,
 	else if (Matches("ALTER", "DOMAIN", MatchAny))
 		COMPLETE_WITH("ADD", "DROP", "OWNER TO", "RENAME", "SET",
 					  "VALIDATE CONSTRAINT");
+	/* ALTER DOMAIN <sth> ADD */
+	else if (Matches("ALTER", "DOMAIN", MatchAny, "ADD"))
+		COMPLETE_WITH("CONSTRAINT", "NOT NULL", "CHECK (");
+	/* ALTER DOMAIN <sth> ADD CONSTRAINT <sth> */
+	else if (Matches("ALTER", "DOMAIN", MatchAny, "ADD", "CONSTRAINT", MatchAny))
+		COMPLETE_WITH("NOT NULL", "CHECK (");
 	/* ALTER DOMAIN <sth> DROP */
 	else if (Matches("ALTER", "DOMAIN", MatchAny, "DROP"))
 		COMPLETE_WITH("CONSTRAINT", "DEFAULT", "NOT NULL");