Integrate fences_block as submodule

Problem/Motivation

Fences Block → was a separate module so far, but to be better integrated, receive current changes and provide support for all features, I'd vote to integrate it as submodule.

This should also work for layout_builder blocks in layout builder and of course regular blocks.

@droath as maintainer of fences_block should also become maintainer of fences, if he's still active.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes