Use a child of FieldConfigEditForm when adding a new field

For now, the child class does not override anything in the parent. As long as we work with the child class, we should not break anything having to do with editing existing fields.

Closes #3386762

core/modules/field_ui/src/Controller/FieldConfigAddController.php

@@ -59,7 +59,7 @@ public function fieldConfigAddConfigureForm(string $entity_type, string $field_n
       'field_storage' => $temp_storage['field_storage'],
     ]);
 
-    return $this->entityFormBuilder()->getForm($entity, 'default', [
+    return $this->entityFormBuilder()->getForm($entity, 'add', [
       'default_options' => $temp_storage['default_options'],
     ]);
   }