message: fix(types): add types for Vue.util.warn function
  See
diff: @@ -121,6 +121,10 @@ export interface VueConstructor<V extends Vue = Vue> {
  observable<T>(obj: T): T;
  + util: {
  + warn(msg: string, vm?: InstanceType<VueConstructor>): void;
  + };
  +
  config: VueConfiguration;
  version: string;
  }
  