[nvim] Fix floating window requirements (#2089)

Vim 8.1.2371
https://github.com/vim/vim/commit/05ad5ff0ab34ed9a5296dedd420ca81698b8ce22

Nvim 0.4.0
https://github.com/neovim/neovim/commit/9a1675b065394734ddaef91a314896028e2b1d46

plugin/fzf.vim

@@ -652,8 +652,8 @@ function! s:split(dict)
   try
     if s:present(a:dict, 'window')
       if type(a:dict.window) == type({})
-        if !has('nvim') && !has('patch-8.2.191')
-          throw 'Vim 8.2.191 or later is required for pop-up window'
+        if !(has('nvim') ? has('nvim-0.4') : has('popupwin') && has('patch-8.2.191'))
+          throw 'Nvim 0.4+ or Vim 8.2.191+ with popupwin feature is required for pop-up window'
         end
         call s:popup(a:dict.window)
         let is_popup = 1