Allow extensions to position new config vars in an array

The core event core.acp_board_config_edit_add only allows extensions to edit or add new config vars to an array. However, if an author wants to put their config vars in a certain position in the array so they will display in a logical place in the ACP, the author must devise their own array slice and dice strategy.

phpBB should provide a function they can use to easily position their new config array before or after a targeted core config var name.

e.g.:

$display_vars['vars'] = insert_config_array($display_vars['vars'], $custom_config_vars, array('after' => 'config_name'));

Inspired by this topic: https://www.phpbb.com/community/viewtopic.php?f=461&t=2247396