Add a position argument to the `add_submenu_page` function similar to the one already in `add_menu_page`. When adding sub menus enables setting the position in the sub menu where the item should appear.
In addition, add the position argument to functions that call `add_submenu_page` under the hood: `add_management_page`, `add_options_page`, `add_theme_page`, `add_plugins_page`, `add_users_page`, `add_dashboard_page`, `add_posts_page`, `add_media_page`, `add_links_page`, `add_pages_page` and `add_comments_page`.
Props welcher, birgire, alexvorn2.
Fixes#39776.
git-svn-id: https://develop.svn.wordpress.org/trunk@46197 602fd350-edb4-49c9-b593-d223f7449a82
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