Add ability to navigate to/from docks via keybindings (#7141)

This adds the ability to navigate to/from docks (Terminal, Project,
Collaboration, Assistant) via keybindings.

When using the `ActivatePaneInDirection` keybinding from the
left/bottom/right dock, we check whether the movement is towards the
center panel. If it is, we focus the last active pane.

Fixes https://github.com/zed-industries/zed/issues/6833 and it came up
in a few other tickes/discussions.

Release Notes:

- Added ability to navigate to docks and back to the editor using the
`workspace::ActivatePaneInDirection` action (by default bound to `Ctrl-w
[hjkl]` in Vim mode).
([#6833](https://github.com/zed-industries/zed/issues/6833)).

## Drawback

There's this weird behavior: if you start Zed and no files are opened,
you focus terminal, go left (project panel), then back to right to
terminal, the terminal isn't focused. Even though we focus it in the
code.

Maybe this is a bug in the current focus handling code?

## Demo


https://github.com/zed-industries/zed/assets/1185253/5d56db40-36aa-4758-a3bc-7a0de20ce5d7

---------

Co-authored-by: Piotr <piotr@zed.dev>