A page attributes panel slot now exists

[philwp]

What?

Adding a Slot on the Page Attributes Panel

Why?

This resolves #31888 and allows plugin authors to extend that panel with additional information.

How?

Added a new Slot PluginPageAttributesPanel which adds a new panelRow below the panel items from core. This is essentially mimicking the behavior provided by the page_attributes_misc_attributes hook.

Testing Instructions

1. Register a plugin

const MyPageAttributes = () => {
    return (
        <PluginPageAttributesPanel className="different">
			<p>
				{ __(
					'Dude',
					'gutenberg-slot-fill-system'
				) }
			</p>
		</PluginPageAttributesPanel>
    );
}
// Register the plugin.
registerPlugin( 'my-page-attributes', { render: MyPageAttributes } );

2. See that it appears in the page attributes panel in the editor while editing a page.

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

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Co-authored-by: philwp <philwebs@git.wordpress.org>
Co-authored-by: colorful-tones <colorful-tones@git.wordpress.org>
Co-authored-by: fabiankaegy <fabiankaegy@git.wordpress.org>

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[colorful-tones]

"Phil's Fill" love it! 😆