Social Links: Allow icon size to be reset and honor theme.json styles

[karthikeya-io]

What?

Closes #70318

This PR updates the SocialLinksEdit component to:

• Introduce a "Default" option for icon size in the toolbar dropdown.
• Ensure the "Default" state maps to undefined in the size attribute instead of hardcoding has-normal-icon-size
• Ensure that resetting the size removes the has-normal-icon-size class.

Why?

Currently, when the icon size is set and reset, the has-normal-icon-size class remains in the markup. This overrides custom styles set via theme.json or additional CSS due to higher specificity.

By introducing an explicit "Default" option that doesn't apply a class, this change ensures the block respects styles defined in theme.json. It also improves clarity in the UI, indicating when the default size is being used.

How?

• Added a "Default" option with an empty string value ('') to the SelectControl options.
• Updated logic to treat this as undefined internally.
• Ensured that selecting or resetting to "Default" results in no size class being written to the block.

Testing Instructions

1. Insert a Social Links block into a post or page.
2. Open the block settings sidebar.
3. Expand the "Icon size" dropdown:

• Confirm that "Default" is now an option.

4. Select other sizes like Small, Normal, Large.

• Confirm the correct class is applied (e.g., .has-small-icon-size).

5. Switch back to "Default" or Reset the size:

• Confirm the class is removed from markup.

6. Add font size overrides in theme.json for the Social Links block.

• Confirm that they now apply correctly when "Default" is selected.

Testing Instructions for Keyboard

Same

Screenshots or screencast

When the following style is added to theme.json

"core/social-links": {
		"typography": {
			"fontSize": "var(--wp--preset--font-size--x-large)"
		}
	},

After Icon size is reset

Before	After

[github-actions]

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Co-authored-by: karthikeya-io <karthikeya01@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>

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[t-hamano]

LGTM! I believe this is the ideal approach to solve the fundamental problem.