WIP: Cake 4.x

[cschomburg]

This fixes a few breaking changes for Cake 4.x and uses Bootstrap3 as its base. I've updated all the broken return types I could find, and fixed a few docblock mistakes. On a cursory glance, it works as expected in my app, but I didn't yet test all the features.

BC breaks:

• DateTimeWidget is currently broken since Cake 4 migrated to HTML 5 inputs instead of selects. I'm not using this feature, so I probably won't fix it.
• ClassTrait::addClass() drops support for array class arguments to conform to Cake 4
• Deprecated HtmlHelper::getCrumbsList() was removed

[cschomburg]

Oh well, I didn't notice the closed PR #188 and fork at https://github.com/fawno/cakephp4-bootstrap-helpers, so this is probably redundant.

I'll leave this open for visibility, until it's clear whether @Holt59 or @alphp plan to provide a maintained repo for Cake4+BS3.