Map costumeName -> costumenumbername [name]

...instead of mapping to a new obsolete/hidden block 'costumename'.
Thanks to @kchadha for helping with this!

src/serialization/sb2.js

@@ -954,6 +954,12 @@ const parseBlock = function (sb2block, addBroadcastMsg, getVariableId, extension
             value: 'number'
         };
         break;
+    case 'costumeName':
+        activeBlock.fields.NUMBER_NAME = {
+            name: 'NUMBER_NAME',
+            value: 'name'
+        };
+        break;
     }
 
     // Special cases to generate mutations.

src/serialization/sb2_specmap.js

@@ -429,7 +429,7 @@ const specMap = {
         ]
     },
     'costumeName': {
-        opcode: 'looks_costumename',
+        opcode: 'looks_costumenumbername',
         argMap: [
         ]
     },