`register_setting` can now be passed an array arguments to specify meta-data about the setting,
much like using the `register_meta` API. Of note, it will now accept a `show_in_rest` arg to
hint the inclusion of the setting in the REST API. `get_registered_settings()` is available
as a utility to get all registered settings.
Props rmccue, aaroncampbell.
Fixes#37885.
Built from https://develop.svn.wordpress.org/trunk@38635
git-svn-id: http://core.svn.wordpress.org/trunk@38578 1a063a9b-81f0-0310-95a4-ce76da25c4cd