Ever since we started writing the entire settings file out ourselves,
we've had the opportunity to control which schema it uses.

This is a quality-of-life improvement for Preview users, and might make
life easier for Dev users as well.

One thing to consider is whether, if building Terminal in Debug+Dev
mode, we should substitute in a `file:///` path. That would make the
_local_ dev experience way cooler and cost us practically nothing.

… builds