linter: add cross-file validation for NOT NULL constraint pattern

[reteps]

Generally, I want to be able to track the NOT NULL constraints across migration files as our postgres deployments have a transaction-per-file, and so I do the constraint in a different, earlier migration than the SET NOT NULL change.

-- migration 1
ALTER TABLE foo ADD CONSTRAINT foo_bar_not_null CHECK (bar IS NOT NULL) NOT VALID;

-- migration 2
ALTER TABLE foo VALIDATE CONSTRAINT foo_bar_not_null;

ALTER TABLE foo
  ALTER COLUMN bar SET NOT NULL;

ALTER TABLE foo
  DROP CONSTRAINT foo_bar_not_null;

Follow up to #910

[netlify]

👷 Deploy request for squawkhq pending review.

Visit the deploys page to approve it

Name	Link
🔨 Latest commit	3bbd0fa

[sbdchd]

Sorry for the delay, looks good, thank you!