Tree component: multiple items at the root level

It looks like the tree component could only build trees with one root node.

"School Activities" in examples.

In my use case I need to build a tree with multiple items at the root level.
Deque University has such an example: https://dequeuniversity.com/library/aria/tree-view

I tried to workaround the issue, placing multiple components one under another:
```jsx
...
...
```
but this does not produce a valid tree structure and I lost the ability to move around the root nodes.