Add margin block support to Site Title

[stacimc]

Description

Part of #23228

Adds the margin block support to the Site Title block.

How has this been tested?

Manually.

1. Using a blocks theme such as Twenty Twenty-One Blocks, update the theme.json to enable margin support under the defaults or the core/site-title context:

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

2. Open the site editor, and select the Site Title block.
3. Open the Settings sidebar and expand the spacing settings panel in the inspector controls sidebar.
4. Verify that the controls for both Padding and Margin are present.
5. Modify the block's margins and verify that it looks as expected.
6. Save the post and view on the frontend, confirming the Site Title displays correctly.

Screenshots

Types of changes

New feature (non-breaking change which adds functionality)

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).

[aaronrobertshaw]

This works as advertised in the Site Editor.

The Site Title block can also be added within a normal post in the block editor. In that scenario, it might catch people off guard that if not nested within another block the left/right margins are ignored due to !important styles added via the layout block support.

That said, I believe this can be merged. If the situation I mentioned becomes an issue we could look to extend the spacing support to allow different sides config when used as a "top level" block.