Improve focus-back-and-forth: remember the whole focus history stack

[nikitabobko]

Excerpt from the docs:

AeroSpace stores only one previously focused window in history, which means that if you close the previous window, focus-back-and-forth has no window to switch focus to. In that case, the command will exit with non-zero exit code.

That’s why it may be preferred to combine focus-back-and-forth with workspace-back-and-forth:

aerospace focus-back-and-forth || aerospace workspace-back-and-forth

This is bad and should be improved. We should store the whole focus history stack. Optionally making it available for users via smth like list-focus-history-stack (also see: #942)