Update/Install: Trigger a JS event when updating a theme.

For plugins a `'wp-plugin-updating'` event is triggered, for themes there's now the `'wp-theme-updating'` equivalent.

Props DavidAnderson.
Fixes #37216.

git-svn-id: https://develop.svn.wordpress.org/trunk@37970 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/js/updates.js

@@ -886,6 +886,8 @@
 		wp.a11y.speak( wp.updates.l10n.updatingMsg, 'polite' );
 		$notice.text( wp.updates.l10n.updating );
 
+		$document.trigger( 'wp-theme-updating' );
+
 		return wp.updates.ajax( 'update-theme', args );
 	};