Tabs组件实现的侧边栏，如何实现自顶到底的效果

问题现象
在实现侧边栏时，希望侧边栏从最顶部开始往下排列，在条目撑满侧边栏时可以滑动，且在选中条目时能自动居中。但是使用Tabs组件中把页签设置在侧边，页签中的条目始终是居中排列，无法做到从最顶部开始往下排列。

背景知识
Tabs组件：是一种常见的界面导航结构。通过页签容器，用户可以快捷地访问应用的不同模块。
使用Tabs组件时，可以使用barMode(BarMode.Scrollable)配置实现可滚动页签，但是Tabs组件中条目高度无法自定义，而且条目数量未撑满Tabs组件时，无法实现自顶到底的效果。
List组件适合连续、多行呈现同类数据，例如图片和文本。
解决方案
由于Tabs组件无法实现自顶到底的效果，可以通过自定义组件来达到此效果。实现如下：

最外层使用Row()组件包裹，使List自定义的导航栏和Tab页内容左右分布。
导航栏使用List实现，使用Scroller的scrollToIndex方法控制List的滚动，达到子项居中显示的效果。
使用TabsController的changeIndex方法控制Tab组件的切换。
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