Per T222042, it's possible now to enforce trailing commas in the last lines of multiline PHP arrays (with phpcbf including an automatic fix). I propose we enable it by default (add it to the MediaWiki style):
- it makes code style less ambiguous (which is generally a good thing, because there is less room for reviewer subjectivity, and less room for different repos ending up with different style checks);
- it simplifies diffs (only the line you added shows up as a change and not the previous one) and reduces the chance of merge conflicts;
- it makes adding and removing lines at the end of arrays more convenient and less error-prone.