feat(command-panes): allow to start suspended

[imsnif]

This allows command panes (started either through the CLI (eg. with zellij run) through layouts or through keybindings) to start suspended. Meaning that their command will only run once the user presses ENTER on them for the first time, eg.

layout {
    pane command="ls" {
        args "-l"
        start_suspended true
    }
}