What problem does this feature solve?

current vue types export don't has export gloabl namespace. https://cdn.jsdelivr.net/npm/vue@3.0.5/dist/vue.d.ts

What does the proposed API look like?

like this https://github.com/vuejs/vue/blob/dev/types/umd.d.ts#L48

/// <reference path="./node_modules/vue/dist/vue.d.ts" />

Vue. // this will trigger code completion
}