Update fzf.vim - support nushell

[Jasha10]

This closes #4066, adding support for nushell in the fzf plugin.

I've taken care to ensure that behavior for other shells will not be modified.

[junegunn]

You can't pass a list to system and systemlist on regular Vim. Also, when &shell is set to nu, system doesn't work at all on Vim.

set shell=nu
echo system('ls')
  " E282: Cannot read from "..."

It's a known issue: https://github.com/nushell/nushell/issues/8981