ci: #3628635 Add GitLab CI

Adds the standard Drupal GitLab CI template.

Updates the existing functional test to use Stark instead of the unavailable Stable theme.

Issue: https://www.drupal.org/project/glightbox/issues/3628635

Validation: the pipeline passes with advisory warnings. PHPUnit and both Composer jobs pass. Existing PHPStan, PHPCS, ESLint and spelling findings were reviewed and can be addressed separately.

Equivalent GLightbox Media Video contribution: https://www.drupal.org/project/glightbox_media_video/issues/3628628

AI-Generated: Yes (Codex prepared the configuration and test setup adjustment.)