twelve-days: default sing method

exercises/twelve-days/src/main/java/TwelveDays.java

@@ -6,4 +6,8 @@ class TwelveDays {
     String verses(int startVerse, int endVerse) {
         throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
     }
+    
+    String sing() {
+        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
+    }
 }