feat(types): expose $vnode

Author: yyx990803

types/test/vue-test.ts

@@ -14,6 +14,7 @@ class Test extends Vue {
     this.$slots;
     this.$isServer;
     this.$ssrContext;
+    this.$vnode;
   }
 
   // test property reification

types/vue.d.ts

@@ -44,6 +44,7 @@ export declare class Vue {
   readonly $isServer: boolean;
   readonly $ssrContext: any;
   readonly $props: any;
+  readonly $vnode: VNode;
 
   $mount(elementOrSelector?: Element | String, hydrating?: boolean): this;
   $forceUpdate(): void;