StepperController class

Controller for managing stepper state and navigation.

Extends ValueNotifier to provide reactive state updates when the current step changes or step states are modified. Includes methods for navigation (next/previous), direct step jumping, and setting individual step states.

The controller should be disposed when no longer needed to prevent memory leaks.

Example:

final controller = StepperController(currentStep: 0);

// Navigate to next step
controller.nextStep();

// Mark step as failed
controller.setStatus(1, StepState.failed);

// Jump to specific step
controller.jumpToStep(3);

// Don't forget to dispose
controller.dispose();