Add 'User FernInit' autocmd to tell 3rd parties

3rd party plugins can use that autocmd to initialize like

    function! s:init() abort
      " Any autocmd function of fern is available here
      call fern#hook#add(...)
    endfunction

    augroup fern-xxxxx-internal
      autocmd! *
      autocmd User FernInit ++once s:init()
    augroup END

Author: lambdalisue

plugin/fern.vim

@@ -31,4 +31,8 @@ augroup fern_internal
   autocmd! *
   autocmd BufReadCmd fern://* ++nested call s:BufReadCmd()
   autocmd SessionLoadPost fern://* ++nested call s:BufReadCmd()
+  autocmd User FernInit ++once :
 augroup END
+
+" Tell 3rd parties that fern has initialized
+doautocmd <nomodeline> User FernInit