[SuperEditor][SuperReader][Desktop] - Add a controls scope, like mobile

[matthew-carroll]

We've added scopes with controllers for Android and iOS controls. Do something similar for desktop.

Desktop doesn't include any handles, magnifiers, or standard popover toolbars. However, desktop might want to show a caret with an expanded selection, or not. This is something that should be configurable in a way that's similar to configuring mobile handles.

Also, with the introduction of a desktop scope, document layers can check for the existence of such a scope to decide whether to build desktop affordances, like the standard desktop caret.