Spacing support: Add server-side support for margins

[stacimc]

Description

This PR adds the margin block support on the server-side allowing dynamic blocks to use it. The existing padding.php block support file is renamed to spacing to reflect the existing block support's structure.

How has this been tested?

Testing instructions
Your theme will need to opt into padding support. You can do this by updating your theme.json to enable padding under the spacing control, in either your default settings or the block context for button:

"settings": {
    "spacing": {
        "customPadding": true,
        "customMargin": true,
        "units": [
            "px",
            "em",
            "rem",
            "vh",
            "vw"
        ]
    }
}

I used the Site Title block for testing, as it supports both margin and padding on trunk.

Test Instructions

You can verify the bug first on trunk by:

1. Create a post and add a Site Title block.
2. In the Inspector Controls, give the block margin and padding.
3. Save and view on the frontend. You should see that the padding has been applied, but the margin has not. (Inspect the element in the console to easily verify that the margin styles have not been added).

Test the PR:

1. Create a post, add a Site Title block.
2. In the Inspector Controls, give the block margin and padding.
3. Save and view on the frontend. You should now see that both the margin and padding have been applied, and can verify by inspecting the element.

**Note that if added as a top-level block, left and right margins may be overridden by the !important styles from the layout block support. You should still be able to see the styles added.

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[carolinan]

The example theme.json code above also need to include "customMargin": true,.

With that and the PR, the margin that is not reset by the layout setting works well on the font.

[apeatling]

Works well, tested alongside #31809.

[aaronrobertshaw]

Thanks for adding this missed functionality 🙇

This tests well for me both in the editor and frontend.

The e2e test failures looked unrelated. After re-running them everything passes. I'll merge this shortly.