Treegrid in 18.2

Hello,

Since 18.2, `FlatTreeControl`, `MatTreeFlatDataSource` and `MatTreeFlattener` are deprecated, and I was relying on them to create a treegrid, as suggested by @andrewseguin in https://github.com/angular/components/issues/15187#issuecomment-463715842:

> You should be able to do this using the table currently, and to make it easier you can use the `TreeControl` to help you traverse your data and manage expansion state
> https://stackblitz.com/edit/angular-m77g7e-semvxp?file=app%2Ftable-basic-example.html

So I was wondering if there was a new recommended way of creating a treegrid, or if I needed to reimplement those deprecated classes. 🤔

Thanks for your help!

阅读 FlatTreeControl、MatTreeFlatDataSource 和 MatTreeFlattener 的弃用背景，以及从 issue #15187 链接的 treegrid 示例。确认 Angular Material 是否有受支持的替代方案，或记录推荐的 treegrid 路径；完成的标准是受支持的方法已明确。

由索引模型根据 Issue 内容生成。