[vim] Fix version check on windows/powershell (#2894)

- Replace fzf#shellescape with shellescape
- Prepend command with '&' in powershell to deal with quoted exe

plugin/fzf.vim

@@ -164,7 +164,7 @@ function s:get_version(bin)
   if has_key(s:versions, a:bin)
     return s:versions[a:bin]
   end
-  let command = fzf#shellescape(a:bin) . ' --version --no-height'
+  let command = (&shell == 'powershell' ? '&' : '') . shellescape(a:bin) . ' --version --no-height'
   let output = systemlist(command)
   if v:shell_error || empty(output)
     return ''