virtual-scroll: allow setting the scrolled-to index

Add an input to the virtual scroll container that allows developers to set the index of the first element that should be visible in the viewport.

We should also allow specifying an index that is greater than the length of the currently loaded data. Once the data loads we can scroll to the specified index.

首先定位虚拟滚动容器的入口点，并了解当前 viewport 索引是如何管理的。跟踪数据加载路径，以了解如何保留一个超出已加载数据范围的请求索引。完成标准是开发者可以设置第一个可见索引，并且容器会在所需数据加载完成后滚动到该索引。

由索引模型根据 Issue 内容生成。