[1.17] Add support for multiple schemes to the SUI

> _Hmm._ I think it would be better to handle this in the viewmodel. Right now a reader needs to keep in their head that the setting is displayed if there's a Dark color scheme, but then it just _edits_ `CurrentColorScheme`. What does that mean?
>
> IMO, "Has Dark" and "Edit Both" should be hidden as implementation details inside the VM.
>

_Originally posted by @DHowett in https://github.com/microsoft/terminal/pull/14064#discussion_r1023275324_

Locate the SUI and its viewmodel, then trace how Has Dark, Edit Both, and CurrentColorScheme are currently represented and used. Clarify the expected multiple-scheme behavior from the linked discussion; done should leave those implementation details inside the viewmodel while the SUI exposes the appropriate setting.

Written by the indexing model from the issue text.