Fixes:
https://linear.app/zed-industries/issue/Z-2680/add-a-close-all-docks-action

I frequently get stuck in this state:

<img width="1608" alt="SCR-20230721-dgvs"
src="https://github.com/zed-industries/zed/assets/19867440/13257e6d-f75a-4d1c-9718-153499e90c60">

I could zoom, but I dont want to in this case, I just want to close
everything, to get back to a truly decluttered state. Running 3 toggle
commands is cumbersome. I'd like to be able to close all docks with one
action.

I added an action with the key binding `alt-cmd-y` (similar
to`alt-cmd-t`, which is used to close all tabs). My original choice was
`alt-cmd-d` (`d` for dock), but that is the default macOS key binding to
hide the system dock.


Release Notes:

- Added a `workspace: close all docks` action (deployed via
`alt-cmd-y`).