Bumping pxt-core to 12.0.15

Author: abchatra

package.json

@@ -46,7 +46,7 @@
     },
     "dependencies": {
         "pxt-common-packages": "13.0.0",
-        "pxt-core": "12.0.13"
+        "pxt-core": "12.0.15"
     },
     "overrides": {}
 }