chore: remove box-shadow divider and update focus ring in TableView

[yihuiliao]

Some nice-to-haves that we discussed during testing. Also follows up on one comment that Devon make on the original PR here and simplifies things by removing the use of box-shadow and replacing it with borders instead.

So in short the changes are:

1. Remove the use of box-shadows to render the gray dividers in favor of border
2. Update the focus ring to match ListView. Instead of the focus ring rendering inside the row and flush against the gray dividers, they now render on top of them instead.

https://www.chromatic.com/build?appId=5f0dd5ad2b5fc10022a2e320&number=1200

Not a blocker for release, just some minor improvements

✅ 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:

Go to S2 storybook and sanity check that using borders for the gray dividers did not change anything in highlight selection (like layout changes when you select and deselect a row)

For the focus ring, make sure to test the focus ring for the entire row but also the cell focus ring. It should closely match what ListView does where the focus ring renders on top of the gray dividers rather flush against it

🧢 Your Project:

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs

[snowystinger]

A little confused about the change to TreeView's root dropzone