[Feat]: When dragging an element, don't default to the top layer, you want to have a config item that controls whether it needs to be placed at the top level #6999

Describe the feature / 功能描述

拖拽元素时，不要默认将其置于顶层，希望有个配置项可以控制是否需要置于顶层

改造一下onDragStart方法：

/**
   * <zh/> 拖拽开始时的回调
   *
   * <en/> Callback when dragging starts
   * @param event - <zh/> 拖拽事件对象 | <en/> drag event object
   * @internal
   */
  protected onDragStart(event: IElementDragEvent) {
    this.enable = this.validate(event);
    if (!this.enable) return;

    const { batch, canvas } = this.context;
    canvas.setCursor(this.options!.cursor?.grabbing || 'grabbing');
    this.isDragging = true;
    batch!.startBatch();
    this.target = this.getSelectedNodeIDs([event.target.id]);
    this.hideEdge();

    // 添加一个选项来决定是否置顶元素
    if (this.options.frontElement) {
      this.context.graph.frontElement(this.target);
    }
    if (this.options.shadow) this.createShadow(this.target);
  }

Are you willing to contribute? / 是否愿意参与贡献？

✅ Yes / 是