feat: add clearMorphAnimationData function

thermion_dart/lib/thermion_dart/thermion_viewer.dart

@@ -269,6 +269,12 @@ abstract class ThermionViewer {
       ThermionEntity entity, MorphAnimationData animation,
       {List<String>? targetMeshNames});
 
+  ///
+  /// Clear all current morph animations for [entity]. 
+  ///
+  Future clearMorphAnimationData(
+      ThermionEntity entity);
+
   ///
   /// Resets all bones in the given entity to their rest pose.
   /// This should be done before every call to addBoneAnimation.