Bump pxt-core to 11.3.15 (#6652)

Author: riknoll

package.json

@@ -32,7 +32,7 @@
     },
     "dependencies": {
         "pxt-common-packages": "12.2.2",
-        "pxt-core": "11.3.13"
+        "pxt-core": "11.3.15"
     },
     "optionalDependencies": {
         "pxt-arcade-sim": "microsoft/pxt-arcade-sim.git#v0.11.12"