feat(primer-miso): request fullscreen mode when expanding eval window

[georgefst]

This is something which I utilised as part of #1350 when testing on mobile. It was handy, for gaining a bit of extra screen space, but it's less of an uncontroversially good idea than those other changes. So I'm just stashing it here as a curiosity to potentially revisit at some point.

• We should really separate the notions of "eval takes up whole app" and "app is actually fullscreen". Those are conflated here. For the latter we should also handle browser events, i.e. that the user can exit fullscreen without using Primer UI (e.g. esc key, in Firefox at least).
• The JSaddle code isn't very idiomatic, and has no error handling.
• In an ideal world, this wouldn't be needed at all, and mobile browsers would just have a generic option for this, like pressing f11 on desktop. To my surprise, only Opera Mini appears to have this, and even there it's clunky.