Motivated by user feature requests

* https://github.com/zed-industries/zed/issues/6880
* https://discord.com/channels/869392257814519848/1439453067119562793

In analogy with VSCode functionality, we're adding a keybinding to the
project panel.

This is particularly for useful for large monorepos.

Release Notes:

- Keybinding added for `CollapseAllEntries` when in the `ProjectPanel`.

Co-authored-by: mikayla <mikayla@zed.dev>