Administration: Relax the default value check for the `$position` argument...

Administration: Relax the default value check for the `$position` argument added to `add_submenu_page()` and related functions in [46197].

Due to a confusion with `add_menu_page()`, which takes the `$icon_url` parameter, while `add_submenu_page()` does not, some plugins were passing in a string instead of integer as `$position`, causing backward compatibility issues.

A `_doing_it_wrong()` message is now added to alert developers of the wrong parameter type.

Props david.binda, desrosj, 123host, dennis_f, MattyRob.
Reviewed by desrosj.
Fixes #48249.
Built from https://develop.svn.wordpress.org/trunk@46570


git-svn-id: https://core.svn.wordpress.org/trunk@46367 1a063a9b-81f0-0310-95a4-ce76da25c4cd