Site Health: Support custom sub-menus and pages.

Allow developers to extend the Site Health screen with their own custom navigation tabs and pages.

This implements a new filter, `site_health_navigation_tabs`, which takes an associated array of tab identifiers/slugs, and tab labels, allowing developers to add their own subpage to the Site Health interface as new tabs.

To output a custom page, or add to an existing page, the `site_health_tab_content` action is triggered whenever the `tab` query argument is present and not empty. This action includes the current tab as its argument, which a developer would match against to only output content when relevant.

Props ramiy for initial patch.
Fixes #47225.

git-svn-id: https://develop.svn.wordpress.org/trunk@50764 602fd350-edb4-49c9-b593-d223f7449a82

src/js/_enqueues/admin/site-health.js

@@ -443,4 +443,9 @@ jQuery( function( $ ) {
 			recalculateProgression();
 		}
 	}
+
+	// Trigger a class toggle when the extended menu button is clicked.
+	$( '.health-check-offscreen-nav-wrapper' ).on( 'click', function() {
+		$( this ).toggleClass( 'visible' );
+	} );
 } );