[feature] strict Promise configuration validation

- updates configuration schema to use `"instanceof": "Promise"` (webpack#258)

depends on (webpack/webpack#6430)

schemas/webpackConfigurationSchema.json

@@ -13,7 +13,7 @@
       }
     },
     {
-      "instanceof": "Function",
+      "instanceof": "Promise",
       "description": "A promise that resolves with a configuration object, or an array of configuration objects."
     }
   ]