Add age-old backward compatibility pointer.

From Canvas of old to Web of today.
Maybe it saves someone a few hours of its life
when porting from pre-0.12.3.

local.amd.json

@@ -11,7 +11,8 @@
     "map": {
         "*": {
             "00comment": "These are backward compatibility pointers.",
-            "amber_core/Web": "amber-contrib-web/Web"
+            "amber_core/Web": "amber-contrib-web/Web",
+            "amber_core/Canvas": "amber-contrib-web/Web"
         }
     }
 }