add onDispose method to ThermionViewer to register callbacks on disposal

thermion_dart/lib/thermion_dart/thermion_viewer.dart

@@ -46,7 +46,6 @@ class TextureDetails {
 }
 
 abstract class ThermionViewer {
-  
   Future<bool> get initialized;
 
   ///
@@ -701,6 +700,11 @@ abstract class ThermionViewer {
   ///
   ///
   AbstractGizmo? get gizmo;
+
+  ///
+  /// Register a callback to be invoked when this viewer is disposed.
+  ///
+  void onDispose(Future Function() callback);
 }
 
 ///
@@ -746,6 +750,7 @@ abstract class Scene {
   ///
   ///
   void registerEntity(ThermionEntity entity);
+
 }
 
 abstract class AbstractGizmo {