Issue #3263873 by ljcarnieri, longwave: drupal.editor.admin needs declare...

Issue #3263873 by ljcarnieri, longwave: drupal.editor.admin needs declare dependency for core/underscore

(cherry picked from commit 97ff906a)

core/modules/editor/editor.libraries.yml

@@ -7,6 +7,7 @@ drupal.editor.admin:
     - core/once
     - core/jquery.once.bc
     - core/drupal
+    - core/underscore
 
 drupal.editor:
   version: VERSION