refactor(iter): move helper functions under vim.iter

vim.iter is now both a function and a module (similar to vim.version).

runtime/lua/vim/_init_packages.lua

@@ -55,6 +55,7 @@ vim._submodules = {
   inspect = true,
   version = true,
   fs = true,
+  iter = true,
 }
 
 -- These are for loading runtime modules in the vim namespace lazily.