lua/stdlib: Introduce vim.shared

This is where "pure functions" can live, which can be shared by Nvim and
test logic which may not have a running Nvim instance available.

If in the future we use Nvim itself as the Lua engine for tests, then
these functions could be moved directly onto the `vim` Lua module.

closes #6580

test/functional/helpers.lua

@@ -846,7 +846,7 @@ local module = {
   window = window,
   winmeths = winmeths,
 }
-module = global_helpers.map_extend('error', module, global_helpers)
+module = global_helpers.tbl_extend('error', module, global_helpers)
 
 return function(after_each)
   if after_each then