zed: Add ability to restore last session w/ multiple windows (#14965)

This adds the ability for Zed to restore multiple windows after
restarting. It's now the default behavior.

Release Notes:

- Added ability to restore all windows that were open when Zed was quit.
Previously only the last used workspace was restored. This is now the
default behavior. To get back the old behavior, add the following to
your settings: `{"restore_on_startup": "last_workspace"}` (Part of
[#4985](https://github.com/zed-industries/zed/issues/4985) and
[#4683](https://github.com/zed-industries/zed/issues/4683))

Demo:



https://github.com/user-attachments/assets/57a375ec-0c6a-4724-97c4-3fea8f18bc2d

---------

Co-authored-by: Nathan <nathan@zed.dev>