feat: Expose onFinish and onDismiss listeners

[memishood]

Description

Currently, the onFinish and onDismiss callbacks can only be defined when ShowcaseView is registered, which is often at a global level. This forces developers to handle showcase completion logic far from the widget that initiated it, leading to tightly coupled code and state management challenges.

This change introduces a listener pattern, allowing any widget to subscribe to showcase events dynamically.

• Added addOnFinishCallback and removeOnFinishCallback to allow for dynamic subscription to the showcase completion event.
• Added addOnDismissCallback and removeOnDismissCallback for the showcase dismissal event.
• Updated the core logic to iterate and invoke all registered listeners when a showcase is finished or dismissed, ensuring backward compatibility with the original global callbacks.
• Ensured that all listener lists are cleared when ShowcaseView is unregistered to prevent memory leaks.

Usage Example

class MyFeatureScreen extends StatefulWidget {
  // ...
}

class _MyFeatureScreenState extends State<MyFeatureScreen> {
  void _onShowcaseFinished() {
    // Handle logic after the showcase
  }

  void _onShowcaseDismissed(GlobalKey? dismissedAt) {
    // Handle logic when showcase is dismissed
  }

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addPostFrameCallback((_) {
      ShowcaseView.get().addOnFinishCallback(_onShowcaseFinished);
      ShowcaseView.get().addOnDismissCallback(_onShowcaseDismissed);
    });
  }

  @override
  void dispose() {
    ShowcaseView.get().removeOnFinishCallback(_onShowcaseFinished);
    ShowcaseView.get().removeOnDismissCallback(_onShowcaseDismissed);
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    // ...
  }
}

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

Closes #555

[aditya-css]

Great work! @memishood. Thanks for raising this PR.