Add filter to custom service icon

[yannkozon]

What?

Add filter to custom service icon for social-link blocks

Why?

A simply filter for custom service icon

Testing Instructions

function example_callback( $value, $service ) {

	if( $service === 'facebook' ) {
		return '<svg width="24" height="24" ...></svg>';
	}

    return $value;
}
add_filter( 'social_link_icon', 'example_callback', 10, 2 );

[github-actions]

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[skorasaurus]

This is certainly a lot simpler than #30749 although I haven't looked at any of the code or any of the discussion there.

[ChrissiQ]

I believe it is simpler because it is only doing the PHP half of the equation. I think this filter will not affect the block editor, only the front end. So the other PR is more complete.