Add a path variant system

Im currently looking for architectural review, particularly from XB stakeholders, before tests are added.

PHPStan and linting has been much improved, to a level above our formal required minimum level 1.

The change to make path fields multi cardinality impacts serialization and JSONAPI especially, and their related tests.

Some tests are updated, a bunch of random test failures still come up recently..

Some of @larowlans work from ✨ Support adding additional routes for view modes other than 'full' Active contained changes to path UI. I couldnt tell which direction he was going as the wip commit was a bit messed up:

So I went with a details/fieldsets per path variant instead of a major change to make the existing one fieldset to contain a tabular layout. Something to discuss further anyway. Or even improve in a follow up.

Theres something up with the path.module postupdate from 🐛 Improve performance for path_alias queries in a workspace Active . Its not updating the index properly with the variant hook_update at the same time. So I've moved it to a hook_update just for the purposes of getting tests going. I'd appreciate advice there.

From the contrib issue @ ✨ Add path/pathauto integration Active , we might be able to do link templates + associated toUrl bits here.

Would we need safeguards to avoid someone adding "edit" as a view mode, which would lead to two possible route matches for "node/$nid/edit"?