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>
diff --git a/runtime/ftplugin/vim.vim b/runtime/ftplugin/vim.vim
index a8636f4..e28beb9 100644
--- a/runtime/ftplugin/vim.vim
+++ b/runtime/ftplugin/vim.vim
@@ -15,6 +15,8 @@
 let s:cpo_save = &cpo
 set cpo&vim
 
+compiler vimdoc
+
 if !exists('*VimFtpluginUndo')
   func VimFtpluginUndo()
     setl fo< isk< com< tw< commentstring< include< define<