feat: mocks mounting option

[lmiller1990]

We need to allow people to mock things out. Not sure how this will look since we don't know how things will be attached to Vue.prototype in Vue 3 yet (or do we?)

Relevant: https://github.com/vuejs/vue-test-utils/blob/dev/test/specs/mounting-options/mocks.spec.js

[dobromir-hristov]

Attaching to prototype will not be available like before, though there will be a workaround, which as of now is not yet implemented.

Related RFC - vuejs/rfcs#117

[lmiller1990]

Yep, let's see how this works out. Once Evan implements it, we can see how it works and go from there. I wonder if in 3.0 attaching the the prototype will be less common.

[lmiller1990]

We now have mocks