feat(command-panes): optionally allow panes to be closed on exit

[imsnif]

This gives an option both in the cli and for layouts to make a pane exit immediately if its command exited (rather than stick around, show its exit status and allow the user to re-run it with ).

eg. in layouts

layout {
    pane command="htop" {
        close_on_exit true
    }
}

eg. in the cli:

zellij run --close-on-exit -- htop