GitlabCI: Fix ESLINT validation errors

Quick follow-up on this issue:

A few additional commits were added to the current merge request MR!43 with a few comments, see above at #2.

But mostly, at this point:
Last build on MR: https://git.drupalcode.org/issue/colorbox-3468719/-/pipelines/257030

• The ESLINT job now seems to be passing ✅
  ESLINT job: https://git.drupalcode.org/issue/colorbox-3468719/-/jobs/2473085
   
• The PHPUnit Tests are passing as well 🟢 \o/
  PHPUnit job: https://git.drupalcode.org/issue/colorbox-3468719/-/jobs/2473087

We would greatly appreciate if a maintainer or someone with write permission could take a look at ticket's merge request MR!43 and let us know if there would be any more work needed.

Feel free to let us know if you have any questions or concerns on this initial merge request or any aspect of this ticket in general, we would surely be glad to help.
Thanks in advance for your feedback and reviews.

Hi @DYDave,

Thank you for all the work you have done to fix the ESLint errors and refactor the code for modern javascript.
I left some comments in the merge request, but they are all the same (different files): when attaching behaviors, is adding the requirement for context === document going to break colorbox for ajax-loaded content? e.g. in a multi-page view where results are loaded using ajax.

I did not try to test this, and it's possible I missed something where this scenario could still work. Please let me know what you think.

Thanks!