use the new snippet functions in norg snippets

lua/snippets/norg.lua

@@ -1,8 +1,5 @@
 require('core.snippets.shorthands')
-
+require('core.snippets.functions')
-local function file_name(args, parent, user_args)
-  return vim.fn.expand("%:t:r")
-end
 
 return {
   -- header level 1, usually this has the same name as the file