fix(types): add types for Vue.util.warn function (#11964)

See https://github.com/vuejs/vue/blob/v2.6.12/src/core/util/debug.js#L18-L26

types/vue.d.ts

@@ -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;
 }