fix(type): exports $vfm through the type definition file for typescript imports

[Dschungelabenteuer]

Fixes #173

Exporting a $vfm const within the index.d.ts should be sufficient to correctly import

import { $vfm } from 'vue-final-modal'

which currently emits the following error:

Module '"vue-final-modal"' has no exported member '$vfm'. Did you mean to use 'import $vfm from "vue-final-modal"' instead?

[stonega]

Hope can be merged soon.

[Mini-ghost]

Thanks @Dschungelabenteuer 💚

[hunterliu1003]

Hey @Dschungelabenteuer @stonega
This PR is published at vue-final-modal@3.4.4

[stonega]

Thanks! Is it possible to also update v2 version? I am using v2 version with typescript.