Add GitLab CI and composer.json

Closes #3634607.

Adds the gitlab_templates CI file, opted in to the previous major (Drupal 10) with autorun on pushes and MRs, a composer.json with drupal/queue_ui as a dev dependency, and moves test/ to tests/ so PHPUnit discovers the test.

Locally the unit tests pass on Drupal 10.6.20 and 11.4.11.

AI-Generated: Yes (Used Claude Code to prepare the CI file and composer.json and to run the tests locally.)