feat: allow unknown on the is operator

Also make the IS values strict at the Parser level since IS cannot be
parametrized.

test/fixtures/privileges.sql

@@ -159,6 +159,7 @@ GRANT ALL ON TABLE
     , client
     , clientinfo
     , contact
+    , chores
 TO postgrest_test_anonymous;
 
 GRANT INSERT ON TABLE insertonly TO postgrest_test_anonymous;
@@ -213,4 +214,4 @@ DO $do$BEGIN
     GRANT ALL ON TABLE test.car_models_car_dealers_10to20 TO postgrest_test_anonymous;
     GRANT ALL ON TABLE test.car_models_car_dealers_default TO postgrest_test_anonymous;
   END IF;
-END$do$;
+END$do$;