fix: Virtualized Tree DnD drop indicator

[reidbarber]

Fixes drop indicators for "after" an expanded item to be after the last child (was rendering before the first child).

Previously, you would get behavior like this:

tree-dnd-broken.mov

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

Test new story: https://reactspectrum.blob.core.windows.net/reactspectrum/952182bc2a0e9c8a1373918d32c48bb803c7dc8f/storybook/index.html?path=/story/react-aria-components--tree-with-drag-and-drop-virtualized&providerSwitcher-express=false

🧢 Your Project:

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the documentation

[LFDanLu]

Couple things I noticed, but happy to get this in as is for testing:

1. the "before" drop target for Project doesn't seem to be be visible, could be a styling issue perhaps.
2. I'm able to have the drop target set as the collection itself upon starting a keyboard drag operation like below:
   

This seems to be intermittent, but the easiest way I've found to reproduce it is by doing the following:

1. Start a keyboard drag for Projects
2. Cancel it via Escape
3. Scroll the Tree down and back up via your touchpad/mouse wheel
4. Start the keyboard drag again for Projects

Looks like maybe the drop indicators past the visible view aren't being persisted upon the start of the second drag?