Initial support for client hints

[kentcdodds]

This implements the ideas described in https://github.com/epicweb-dev/epic-stack/blob/main/decisions/05-client-pref-cookies.md

Eventually we'll want locale stuff in here as well I expect. For now, the timezone offset and theme preference are necessary. I would love ideas and suggestions here. I'm going to be adding full theme support (via user preference) based on this before we merge this. In that world, the client hint for the desired theme is only used when the user chooses "system".