feat: allow unknown on the is operator

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

test/fixtures/data.sql

@@ -727,3 +727,6 @@ INSERT INTO test.contact (id,name, clientid) values (1,'Wally Walton',1),(2,'Wil
 
 TRUNCATE TABLE test.clientinfo CASCADE;
 INSERT INTO test.clientinfo (id,clientid, other) values (1,1,'123 Main St'),(2,2,'456 South 3rd St'),(3,3,'789 Palm Tree Ln');
+
+TRUNCATE TABLE test.chores CASCADE;
+INSERT INTO test.chores (id, name, done) values (1, 'take out the garbage', true), (2, 'do the laundry', false), (3, 'wash the dishes', null);

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$do$;

test/fixtures/schema.sql

@@ -2417,3 +2417,9 @@ CREATE TABLE clientinfo (
 , clientid int unique references client(id)
 , other text
 );
+
+CREATE TABLE chores (
+  id int primary key
+, name text
+, done bool
+);