[meta] Canvas / layout builder / block module reconciliation

I added existing issues for blocks and and navigation that were already open.

Layout builder vs. experience builder does not have an issue. There is not much we can do here until experience builder has a stable release, but I think we can think about what it might look like before then.

e.g. we could move layout builder overrides to a dedicated module in contrib. New sites would be encouraged to use the XB entity field integration, which is the first feature that XB will support when it reaches beta. Existing sites would be able to stay on layout_builder_overrides but there will probably be a way for that to co-exist with XB with a just-in-time migration path. This requires splitting the logic out of layout builder itself but we know that not every site using layout_builder uses it.

For layout builder entity display mode configuration, for me that is a harder one because I think that feature lives in core, but it will be a big challenge to move experience builder support for this into core as well as it working completely differently. However if we decouple those decisions, layout builder move it to contrib with a plan to bring things into core from XB later - and this would address the duplication problem (triplication if you include field UI manage display) if nothing else.