feat: Add 'orientation' prop to GridList

[essandhu]

Closes #6928

Adding an orientation prop to the GridList aria component and updating arrow key navigation to respect the new prop. Added new unit tests to verify this behavior and new Storybook controls and an example ("Draggable Grid List Example") so that drag and drop can be tested.

✅ 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:

Steps to replicate the original issue:

1. Open the GridList story in Storybook
2. Set layout to "stack" and note that Arrow Down/Up moves focus between items.
3. Drag and drop is also locked to vertical arrow keys, which doesn't work for horizontal layouts

Testing the new orientation prop:

1. In the GridListExample story, set orientation to "horizontal" and layout to "stack"
2. Verify that Arrow Left/Right now moves focus between items
3. Set layout to "grid" while keeping orientation "horizontal"
4. Verify Arrow Down/Up moves between items in the cross axis and Arrow Left/Right moves in the primary axis
5. Set orientation back to "vertical" and confirm both stack and grid layouts behave the same as before
6. Open the DraggableGridListExample story and set orientation to "horizontal"
7. Focus an item, press Enter to start a drag
8. Verify drop indicators are navigable with Arrow Left/Right
9. Press Enter to complete the reorder and confirm it works
10. Set orientation back to "vertical" and confirm drag and drop still uses Arrow Down/Up as before

🧢 Your Project:

None

[reidbarber]

LGTM