Add search_index as an option for content preview

Closes #3503211

core/modules/node/src/Form/NodePreviewForm.php

@@ -96,7 +96,6 @@ public function buildForm(array $form, FormStateInterface $form_state, ?EntityIn
     // Unset view modes that are not used in the front end.
     unset($view_mode_options['default']);
     unset($view_mode_options['rss']);
-    unset($view_mode_options['search_index']);
 
     $form['uuid'] = [
       '#type' => 'value',