runtime(compiler): add vimdoc

closes: #14459

https://github.com/google/vimdoc generates vim help files from vimscript files

Signed-off-by: Wu, Zhenyu <wuzhenyu@ustc.edu>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/ftplugin/vim.vim

@@ -15,6 +15,8 @@ let b:did_ftplugin = 1
 let s:cpo_save = &cpo
 set cpo&vim
 
+compiler vimdoc
+
 if !exists('*VimFtpluginUndo')
   func VimFtpluginUndo()
     setl fo< isk< com< tw< commentstring< include< define<