feat:scroller add onScroll and onScrollEnd property

doric-js/lib/src/pattern/mvvm.d.ts

@@ -9,6 +9,7 @@ export declare abstract class ViewModel<M extends Object, V extends ViewHolder>
     private viewHolder;
     constructor(obj: M, v: V);
     getState(): M;
+    getViewHolder(): V;
     updateState(setter: Setter<M>): void;
     attach(view: Group): void;
     abstract onAttached(state: M, vh: V): void;

doric-js/lib/src/pattern/mvvm.js

@@ -9,6 +9,9 @@ export class ViewModel {
     getState() {
         return this.state;
     }
+    getViewHolder() {
+        return this.viewHolder;
+    }
     updateState(setter) {
         setter(this.state);
         this.onBind(this.state, this.viewHolder);