Part of https://github.com/zed-industries/zed/issues/14496

This PR adds highlighting on the containing folder which would be the
target of the drop operation. It only highlights those directories where
actual drop is possible, i.e. same directory where drag started is not
highlighted.

- [x] Tests


https://github.com/user-attachments/assets/46528467-e07a-4574-a8d5-beab25e70162

Release Notes:

- Improved project panel to show a highlight on the containing folder
which would be the target of the drop operation.