Content lock for layout builder

Problem/Motivation

Currently, Content Lock in Layout Builder isn't working as expected. Locked content can still be edited and saved by another user, even though the message at the top indicates that the content is locked.

The question is whether to provide a `content_lock_layout_builder` as a submodule, or whether it should be a separate module/project. I've already implemented the basic module and could make it available here.