tab complete for ALTER TABLE ALTER CONSTRAINT

hi.

attached patch is tab complete for
ALTER CONSTRAINT constraint_name [ DEFERRABLE | NOT DEFERRABLE ] [
INITIALLY DEFERRED | INITIALLY IMMEDIATE ] [ ENFORCED | NOT ENFORCED ]
ALTER CONSTRAINT constraint_name [ INHERIT | NO INHERIT ]

based on ALTER TABLE Synopsis section [0]

[0]: https://www.postgresql.org/docs/devel/sql-altertable.html