[vim] Use s:execute_term in Windows

IMPORTANT:
cmd.exe and powershell are fine in default Windows terminal.
cmd.exe prompt is broken on ConEmu because it natively supports ucs-2 only.
utf-16 support is exclusive to .Net (ie. powershell).
utf-8 supports requires chcp, external program, but does not fix the cmd.exe prompt.
Use powershell on ConEmu to avoid corrupted text on display

plugin/fzf.vim

@@ -389,7 +389,7 @@ try
   let use_height = has_key(dict, 'down') &&
         \ !(has('nvim') || s:is_win || has('win32unix') || s:present(dict, 'up', 'left', 'right')) &&
         \ executable('tput') && filereadable('/dev/tty')
-  let use_term = has('nvim-0.2.1') || (has('nvim') && !s:is_win) || (has('terminal') && has('gui_running') && has('patch-8.0.995'))
+  let use_term = has('nvim-0.2.1') || (has('nvim') && !s:is_win) || (has('terminal') && has('patch-8.0.995') && (has('gui_running') || s:is_win))
   let use_tmux = (!use_height && !use_term || prefer_tmux) && !has('win32unix') && s:tmux_enabled() && s:splittable(dict)
   if prefer_tmux && use_tmux
     let use_height = 0