"North up" or other fixed bearing map

A user of the framework noted that some people prefer to keep their maps always "north up." Currently all platforms use MapLibre's built-in user tracking, but it doesn't strictly have to be this way.

In the context of the web (original discussion), the camera bearing is being set here: https://github.com/stadiamaps/ferrostar/blob/03e26754f4c35e7e2991ac19961a8e31f61300a9/web/src/ferrostar-map.ts#L423.

I think we could model this as an enum with values like track course, north up, or something along those lines... (Maybe "fixed bearing" is better than "north up?" Does anyone want to fix to, idk, east?).

Opening this issue to track discussion.