Replace a few forgotten obsolete helpers

- Apparently, I forgot that `array_flatten` comes from Laravel. :)
- When I did this previously, I did not search the views directory.

src/Extension/Extension.php

@@ -267,7 +267,7 @@ class Extension implements Arrayable
             $extenders = [$extenders];
         }
 
-        return array_flatten($extenders);
+        return Arr::flatten($extenders);
     }
 
     /**