types: add UMD global declaration to index.d.ts (vuejs#7868)

Users using Vue from e.g. the CDN might be using the global `.Vue` without importing the module.

See https://www.typescriptlang.org/docs/handbook/modules.html for details on the syntax

Author: RyanCavanaugh

types/index.d.ts

@@ -2,6 +2,8 @@ import { Vue } from "./vue";
 
 export default Vue;
 
+export as namespace Vue;
+
 export {
   CreateElement,
   VueConstructor