Add zellij display mode

[noahbald]

It would be nice if an option similar to fzf --tmux was added for zellij, to display a floating pane for those using zellij instead of tmux

[junegunn]

I like the idea. It would be nice if fzf could automatically choose tmux popup or zellij floating pane when generic --popup option is given.

It would be trivial to implement it if zellij run provided an option to block until completion similarly to tmux display-popup, but unfortunately it doesn't (zellij-org/zellij#3018), so we would need another mechanism (such as using named pipes).