api: Consider behavior of `find` and `findAll`

[lmiller1990]

• in VTU beta these can be called with find(Component)
• this is not supported here (yet)
• should we support this? There are known limitations, at least in VTU beta and Vue 2 (cannot find functional components, Vuetify components, see here Some stubbed Vuetify components can't be found with find wrapper method vuejs/vue-test-utils#1272 )
• how should this API look?