Add SwiftUI runtime checks to guard against nested AXScreens

[Clstroud]

@andrewroycarter found that it's easy to nest screens in a parent-child relationship when that's not really how the framework is designed to be used. SwiftUI provides a means for us to add a runtime check via the environment that could output a useful diagnostic to help discover this kind of configuration error during development.

Documentation should also be updated to make this expectation slightly clearer, even though it's already basically stated.