Increase PHPStan level even more

@alexpott thanks for opening and for the initial branch.

I'll dedicate some time to this one today.

OK, fixing all issues on level 6 is more work than I anticipated, so I'm adding a bunch of things to the baseline. We will be at least enforcing more stricty-typed code going forward. PHPStan is happy now, but in the process I seem to have broken a few tests.

Unassigning myself from this ticket for now since I need to switch to other things, but I'll revisit this later in the week if it's still at the same place.

OK, fixing all issues on level 6 is more work than I anticipated, so I'm adding a bunch of things to the baseline.

Yeah that's why I stopped after doing the simple stuff in tests :D...

It is hard when core's docs don't meet the requirements. And yeah adding to the baseline is a great way forward.

@marcoscano we need to be careful with string vs TranslatableMarkup because PHP will automagically cast TranslatableMarkup to strings and then html will be double escaped incorrectly...

Argh right! Sorry I went too fast on these return type changes...
Thanks for fixing them and the update test 🙏
Merged! Will tag a release in a little bit.