Move view controller helpers to Stripe Core.

[nschris-stripe]

Summary

This moves some ViewController helpers used for SwiftUI presentation into StripeCore. I am doing this because we are actively considering using a similar approach to the payment bottom sheet view modifier for the Connect SDK. There is one minor change I made to make findViewController more accurate, called out in a comment.

Demo

Simulator.Screen.Recording.-.iPhone.8.Plus.-.2025-03-19.at.12.32.52.mp4

[nschris-stripe]

Previously the function assumed the only things in the responder chain were UIViewController or UIView. That is not a requirement of the responder chain, so I made this slightly more generic.

Previously looked like this:

func findViewController(for uiView: UIView) -> UIViewController? {
    if let nextResponder = uiView.next as? UIViewController {
        return nextResponder
    } else if let nextResponder = uiView.next as? UIView {
        return findViewController(for: nextResponder)
    } else {
        // Can't find a view, attempt to grab the top most view controller
        return topMostViewController()
    }
}