feat(cdk/scrolling): Add support for nested for loop

### Feature Description

The Angular Material cdk-virtual-scroll-viewport property is not working correctly for nested *ngFor. The viewport is split into two and the scroll does not work as expected. Please assist in listing in viewport with parent and child.

### Use Case

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从 cdk-virtual-scroll-viewport 入口点开始，复现 issue 中嵌套的 *ngFor 示例。调查 viewport 为何被拆分且滚动失败，然后验证父项和子项是否会一起渲染并按预期滚动。

由索引模型根据 Issue 内容生成。