Adding axial top bottom margin to the cover block.

[ramonjd]

What?

Revives #33835 and fixes #41879 by bringing top and bottom margin controls to the cover block:

Why?

Previous attempts to add this control were put in doubt due to the unavailability of the visualizer. It looks like the visualizer is looking better since #40505

How?

Via block.json

Testing Instructions

1. In a new post, create a Cover block. Check that the dimensions panel is there and margin controls are disabled by default.
2. Select margin under the ToolsPanel and adjust the value.
3. Check that your changes are reflected on the published post.
4. Edit the top and bottom margin separately.
5. Behold your ground-breaking results and rejoice!

[glendaviesnz]

I think the improved visualiser, and limiting it to top and bottom margin answers the concerns from the previous PR. The confusion over collapsing margins is certainly made clearer with the visualiser at least showing the user that something is happening even of the blocks are not moving (top cover block below has a 40px bottom margin set, so any changes to gap won't happen until bottom covers top margin exceeds 40px):

collapse.mp4

I did hit a strange glitch with the visualiser when one cover is below another, but haven't managed to work out exactly how to replicate it yet, and probably not related to this PR:

cover-margin.mp4

[glendaviesnz]

After a page refresh I wasn't able to replicate the issue noted above, so unless someone else sees the same thing we can probably ignore for now.

[ramonjd]

Thanks for testing @glendaviesnz !

After a page refresh I wasn't able to replicate the issue noted above, so unless someone else sees the same thing we can probably ignore for now.

I had a go with several cover blocks stacked, and couldn't replicate (at least on my side)

One thing I forgot to confirm is whether we can reduce the margin to 0, which I think was one of the desired outcomes.

👍

[glendaviesnz]

Looks good to me. I think improvements to the visualiser, along with limiting to top and bottom margins for now answers the issues raised in the previous iteration of this.

[mburridge]

Added the Needs Dev Note label in case this needs a dev note (either individual or as part of a "misc" dev note) for WP 6.1 release.