types: add UMD global declaration to index.d.ts (#7868)
types: add UMD global declaration to index.d.ts (#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
vuejs/vue,Jinjiang/vue,Jinjiang/vue,cnchanning/vue,cnchanning/vue,Jinjiang/vue,vuejs/vue,cnchanning/vue,vuejs/vue,vuejs/vue,Jinjiang/vue