Add option to disable global $sanity in v0.10.0

[marekbrze]

Right now Sanity module is added globally and it's a part of the main node_modules webpack chunk. It's more useful to be able to import it locally in components/pages. In the v1 it's the new default behaviour. It would be great to add it to the older v0.10.0 package that it's widely used in Nuxt 2 apps.