Step constructor
      const
      Step({ 
    
- required Widget title,
- WidgetBuilder? contentBuilder,
- Widget? icon,
Creates a Step.
The title is required and typically contains the step name or description.
The contentBuilder is called when this step becomes active to show
detailed content. The icon replaces the default step number/checkmark.
Parameters:
- title(Widget, required): step title or label
- contentBuilder(WidgetBuilder?): builds content when step is active
- icon(Widget?): custom icon for step indicator
Example:
Step(
  title: Text('Account Setup'),
  icon: Icon(Icons.account_circle),
  contentBuilder: (context) => AccountSetupForm(),
);
Implementation
const Step({
  required this.title,
  this.contentBuilder,
  this.icon,
});