#theme system cleanup

Going to postpone on that issue.

I think we need to consolidate the theme hooks first, some like #theme links already have issues to switch to #type item_list with a theme suggestion, which would be cleaner than a new #type links element.

Would be good to get this done, so that in turn we can do 📌 Replace links.html.twig with item-list--links.html.twig Needs work .

We can deprecate variables in preprocess now, so should be able to do that, then actually remove those variables from the template in 12.x

OK let's postpone this on 📌 Remove title and wrapper div from item-list.html.twig Needs work