Separator Block: Add top & bottom margin support

[ryelle]

This adds top & bottom margin support to the separator block. There have been other attempts at this in the past, see #20758 #30609 #28451. They've been closed in favor of using the spacer block, but I don't think that's intuitive, especially as there are other blocks with margin controls.

For example, if I want to add a spacer with 50px space on either side, I need to add a group block, set the block gap & margin top to 0px (to account for added margins), and then add my spacers & separator.

<!-- wp:group {"style":{"spacing":{"blockGap":"0px","margin":{"top":"0px"}}},"layout":{"inherit":true}} -->
<div class="wp-block-group" style="margin-top:0px">
	<!-- wp:spacer {"height":"50px"} -->
	<div style="height:50px" aria-hidden="true" class="wp-block-spacer"></div>
	<!-- /wp:spacer -->

	<!-- wp:separator {"color":"light-grey","className":"is-style-wide"} -->
	<hr class="wp-block-separator has-text-color has-background has-light-grey-background-color has-light-grey-color is-style-wide" />
	<!-- /wp:separator -->

	<!-- wp:spacer {"height":"50px"} -->
	<div style="height:50px" aria-hidden="true" class="wp-block-spacer"></div>
	<!-- /wp:spacer -->
</div>
<!-- /wp:group -->

Or, I could just control the margin directly.

<!-- wp:separator {"style":{"spacing":{"margin":{"top":"50px","bottom":"50px"}}},"backgroundColor":"black","className":"is-style-wide"} -->
<hr class="wp-block-separator has-text-color has-black-color has-alpha-channel-opacity has-black-background-color has-background is-style-wide" style="margin-top:50px;margin-bottom:50px"/>
<!-- /wp:separator -->

How?

This simply toggles the block support for spacing.margin, for top & bottom margin only (the minimum needed change).

Testing Instructions

1. Add a separator block
2. Change the margin (might need to enable "margin" in the Dimension panel)
3. It should reflect the new margin in the editor
4. Save & view the post
5. The margin should also be applied on the frontend

Screenshots

Added Dimension panel & margin control

Spacer with margin

[nendeb]

In the following use case
If a Separator Block is placed not only between the top and bottom blocks, but also between the left and right blocks, the left and right margins may be necessary as well.

[antonyjsmith]

Discovered what appears to be a bug on this here: #47059 the global margin is not working when set via global styles