Add Gitlab CI support

Oops pushed this in the 2.x branch instead of in the MR branch.
Pipeline does not seem to run automatically though. Will have to figure out what's wrong.

there is a https://git.drupalcode.org/project/ajax_loader/-/pipelines/146942, so something works

Finally, the pipeline is green, but the changes should be well-checked

Thanks for your work @EvilArgest. I try to review the changes soon.

@EvilArgest After almost half a year, I finally reviewed your changes. Thanks for your work.

Automatically closed - issue fixed for 2 weeks with no activity.