Add TypeScript definition for ssrContext (#5654)

Author: zyml

types/test/vue-test.ts

@@ -13,6 +13,7 @@ class Test extends Vue {
     this.$refs;
     this.$slots;
     this.$isServer;
+    this.$ssrContext;
   }
 
   // test property reification

types/vue.d.ts

@@ -42,6 +42,7 @@ export declare class Vue {
   readonly $slots: { [key: string]: VNode[] };
   readonly $scopedSlots: { [key: string]: ScopedSlot };
   readonly $isServer: boolean;
+  readonly $ssrContext: any;
   readonly $props: any;
 
   $mount(elementOrSelector?: Element | String, hydrating?: boolean): this;