[02] Add "final" to foldLeft to allow tailrec check

src/main/scala/datastructures/List.scala

@@ -20,7 +20,7 @@ enum List[+A]:
   /** recurses through the list, combining elements with the given function
    *  Uncomment the annotation to enable checking for tail-recursiveness */
   //@annotation.tailrec
-  def foldLeft[B](z: B)(f: (B, A) => B): B = ???
+  final def foldLeft[B](z: B)(f: (B, A) => B): B = ???
 
 object List:
   /** construct a list by passing elements