Enable testing with GitLab CI

I set up GitLab CI. You can see the results on the MR: https://git.drupalcode.org/project/conditional_fields/-/merge_requests/41.

I fixed most of the errors and warnings reported by phpcs.

I added a baseline for phpstan, so that we can try to avoid new errors and eventually fix the existing ones. I used the one generated as an artifact from the GitLab CI run. But that does not seem to be working, so I am setting the issue status to NW.

I fixed the one problem reported by stylelint.

I would rather let someone more familiar with JS take a look at the eslint results.

I am still doing something wrong with phpstan.

@D-XPERT:

Thanks for looking at the eslint errors. I left a few comments on the MR. If you have a chance, please respond to them. I am setting the issue status back to NW for that.

Your commit messages were brief. Could you say a little more? It looks as though the first commit might have been automated fixes, and the second commit was manual. Is that right?

Meanwhile, I have ignored the remaining phpcs error, fixed my broken phpstan configuration, and updated a test to match the changes in the code. I think this issue is almost done!

@benjifisher, thanks for the review. I will work on the suggestions.

Awesome work in here. 1+! Thanks for working on this important issue! Will follow and will be available for any question.