Added a possibility to remove webview handlers

[dhuebner]

• BREAKING: onRequest() and onNotification() now return Disposable instead of Messenger - This change removes method chaining but provides consistent disposal pattern
  • onRequest<P, R>(type: RequestType<P, R>, handler: RequestHandler<P, R>): Disposable
  • onNotification<P>(type: NotificationType<P>, handler: NotificationHandler<P>): Disposable
  • Migration: Replace method chaining like messenger.onRequest(type, handler).start() with separate calls: messenger.onRequest(type, handler); messenger.start();
• BREAKING: Removed onRequestDisposable() and onNotificationDisposable() methods - These are no longer needed since onRequest() and onNotification() now return disposables directly

New Features

• NEW: unregisterHandler(method: string): boolean - Programmatically unregister message handlers by method name
  • Returns true if handler was successfully removed, false if no handler existed
  • Enables dynamic handler management and cleanup

See #51

[spoenemann]

Looks good, thanks.