make ListCollection and TreeCollection satisfy the Collection interface

[nishitaniyuki]

Currently, ListCollection and TreeCollection does not satisfy the Collection interface, because getKeyBefore() and getKeyAfter() could return undefined. The details are as follows.

Collection interface:

• getKeyBefore() and getKeyAfter() return Key or null
• Key is string or number

ListCollection:

• node.prevKey andnode.nextKey can be undefined
• ListCollection.getKeyBefore() and ListCollection.getKeyAfter() can return undefined

TreeCollection:

• node.prevKey and node.nextKey can be undefined
• TreeCollection.getKeyBefore() and TreeCollection.getKeyAfter() can return undefiend

For example, if we replace state.collection.getKeyAfter(item.key) == null in useDraggableCollection() docs with state.collection.getKeyAfter(item.key) === null, things does not work as expected (according to the type definition this is valid).

✅ 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:

🧢 Your Project: