Allow a nav controller to be pushed into a modal stack

[olivaresf]

I found a scenario we hadn't considered with TurboNavigator: there are times where pushing/replacing the modal controller with a navigation controller is expected. For example, presenting an [EKEventEditViewController](https://developer.apple.com/documentation/eventkitui/ekeventeditviewcontroller) so the user can add a new event to their calendar is currently not possible because EKEventEditViewController inherits from UINavigationController.

With the current implementation, TurboNavigator crashes when trying to push a EKEventEditViewController on a modal stack because it's disallowed to set a navigation controller as part of another navigation controller's view controllers (modalNavigationController.setViewControllers([controller], animated: true)).

I'm not particularly fond of the fix but I also don't see anything obviously wrong with it.

[olivaresf]

The code is ready, but I've turned this into a draft because I'd like to get opinions before moving forward.

[joemasilotti]

This approach sounds solid to me. Checking for inheritance from UINavigationController doesn't seem much different than the check for UIAlertController we are already doing elsewhere.

My only request is to pull out the duplicated test setup code for making a window and moving the no longer private helper somewhere else.

[svara]

Unfortunately this doesn't work when navigating with a UINavigationController instance (e.g. UIImagePickerController, or EKEventEditViewController) while in a modal presentation.

[svara]

Thanks for bringing this up @olivaresf! Navigating with an instances of UINavigationController is a specific case, and we should definitely handle it.
The changes in this PR don't seem to be working for me.

A tangential issue I also ran into is that the current implementation doesn't allow for more than one modal. If we are on a modal screen, and the new screen requires a modal presentation, it gets pushes instead of being presented modally on the modal navigation controller.

I'll provide my feedback in an upcoming PR.

[svara]

I'm looping @jayohms into this discussion.

[olivaresf]

I've discussed this again with @svara and @zoejessica and for now I've decided to close this PR. We don't really have a consensus on whether this solution is moving in the right direction. Even though this is the same as the alert controller check, I'd rather we move more towards these checks being part of the path config somehow. If we have a need for this again, we can re-explore with more time.

Thanks!