[2833] Make layout transitions behave more like page transitions (definition in layout files, function)

Right now page transitions can either be defined globally in the config, or individually in page components. The latter enables two huge features:

• Multiple transitions for different pages
• Access to the Vuex store for more conditional transitions

Layout transitions, however, can only be defined globally in nuxt.config, which limits us to only one transition, and no access to the Vuex store.

Secondly, the page transition property can be a function that conveniently gives us access to the to/from of route changes. This would also be useful in layout transitions.

Tagging @homerjam since you implemented layout transitions 👍