project panel: Add Duplicate action (#12081)

This fixes #5304 by adding a new Duplicate action to the project panel
context menu.

It really is implemented on top of copy&paste.



Release Notes:

- Added a Duplicate action to the project panel.
([#5304](https://github.com/zed-industries/zed/issues/5304)).



https://github.com/zed-industries/zed/assets/1185253/f0fa6a4b-f066-47df-84f0-257a049800d1