bump pxt-core to 12.1.12 (#6615)

Author: riknoll

package.json

@@ -46,7 +46,7 @@
     },
     "dependencies": {
         "pxt-common-packages": "13.0.0",
-        "pxt-core": "12.0.1"
+        "pxt-core": "12.1.12"
     },
     "overrides": {
     }