Add Composer metadata and drupal.org GitLab CI

CI could not install anything: the project had no composer.json, so the GitLab template expanded an empty one and Canvas was never installed. Requiring drupal/canvas ^1.3 is enough for the template to bring it in -- 1.3.0 is the first release with CanvasKernelTestBase's CANVAS_KERNEL_TEST_MINIMAL_MODULES, which the existing kernel test uses; every other Canvas API this module calls has been there since 1.0.0.

core_version_requirement goes from ^11 to ^11.3, the release in which core's SDC metadata learned the noUi flag these definitions set.

drupal/custom_elements is a dev dependency only so that static analysis can run: Canvas's canvas_headless submodule declares it as a dependency without requiring it through Composer, and PHPStan's Drupal autoloader loads every service class in the code base.

The pipeline is the stock contrib template with the validation jobs made non-advisory. Only the current core version is tested, because Canvas cannot be installed on the previous minor, the previous major or the next major.

Closes #3615617