[UIAlertController] Add preferred button support

[juliensagot]

UIKit exposes preferredAction as the way to mark the action a person is most likely to take from an alert.
Per Apple's documentation, UIKit gives that action additional emphasis and lets the Return key trigger it when a physical keyboard is connected.

• Adds ButtonState.isPreferred and .preferred(_:).
• Preserves preferredness through ButtonState.map, equality, hashing, and custom dump output.
• Updates UIKit UIAlertController bridging for both AlertState and ConfirmationDialogState to set preferredAction.
• Reports an issue when multiple buttons are marked preferred, while deterministically using the first one.
• Adds focused value-level and UIKit tests.

Example

state.applyChangesToRoutineDialog = ConfirmationDialogState(
  titleVisibility: .visible,
  title: {
    TextState("Update Routine?", bundle: .module)
  },
  actions: {
    ButtonState(action: .onlyReplaceTrainingExercise) {
      TextState("Training Only", bundle: .module)
    }
    .preferred() // 🟢🟢🟢 This is the new API! 🟢🟢🟢

    ButtonState(action: .replaceBoth) {
      TextState("Update Both", bundle: .module)
    }
  },
  message: {
    TextState(
      "Should the linked routine also be updated with these exercise changes?",
      bundle: .module
    )
  }
)