Add Gitlab CI for automated testing

I can't make past the "composer" step for some reason: https://git.drupalcode.org/project/radiostoslider/-/merge_requests/3

If anyone knows how to fix this please let me know, I'll give you full credits !!!

I fixed, it was a formatting issue, I'll continue with the task.

It seems to be working now, I'll move the task to "Needs Review" and then I'll merge the changes to the 2.0.x branch and move the issue to "Fixed": https://git.drupalcode.org/project/radiostoslider/-/pipelines/429807

Moved to "Fixed"

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