Reset navigation state when popping multiple view controllers

[sk409]

Fixes #323

[areschoug]

Nice fix! This is a test to go with the change that fails on main but works with this fix.

  @MainActor
  func testPushViewController_ManualPopMultiple() async throws {
    let vc = BasicViewController(
      model: Model(pushedChild: Model(pushedChild: Model()))
    )
    let nav = UINavigationController(rootViewController: vc)
    try await setUp(controller: nav)

    await assertEventuallyEqual(nav.viewControllers.count, 3)

    nav.popToViewController(nav.viewControllers[0], animated: false)
    await assertEventuallyEqual(nav.viewControllers.count, 1)
    await assertEventuallyNil(vc.model.pushedChild)
  }