feat(defx): use p to toggle preview windows

close https://github.com/SpaceVim/SpaceVim/issues/4205

config/plugins/defx.vim

@@ -146,8 +146,7 @@ function! s:defx_init()
         \ defx#do_action('drop', 'split')
   nnoremap <silent><buffer><expr> st
         \ defx#do_action('drop', 'tabedit')
-  nnoremap <silent><buffer><expr> p
+  nnoremap <silent><buffer><expr> p defx#do_action('call', g:defx_config_sid . 'DefxPreview')
-        \ defx#do_action('open', 'pedit')
   nnoremap <silent><buffer><expr> K
         \ defx#do_action('new_directory')
   nnoremap <silent><buffer><expr> N
@@ -207,6 +206,27 @@ function! s:DefxSmartL(_)
   endif
 endfunction
 
+function! s:DefxPreview(_) abort
+  if s:preview_windows_opened()
+    pclose
+  else
+    if !defx#is_directory()
+      let filepath = defx#get_candidate()['action__path']
+      exe 'topleft pedit ' . filepath
+    endif
+  endif
+endfunction
+
+fun! s:preview_windows_opened()
+  for nr in range(1, winnr('$'))
+    if getwinvar(nr, "&pvw") == 1
+      " found a preview
+      return 1
+    endif  
+  endfor
+  return 0
+endfun
+
 function! s:DefxSmartH(_)
   " if cursor line is first line, or in empty dir
   if line('.') ==# 1 || line('$') ==# 1