Themes: Introduce register_theme_feature API.

Currently themes can declare support for a given feature by using add_theme_support(). This commit adds a register_theme_feature() API that allows plugins and WordPress Core to declare a list of available features that themes can support.

The REST API uses this to expose a theme's supported features if the feature has been registered with "show_in_rest" set to true.

Props kadamwhite, spacedmonkey, williampatton, desrosj, TimothyBlynJacobs.
Fixes #49406.

Built from https://develop.svn.wordpress.org/trunk@48171


git-svn-id: https://core.svn.wordpress.org/trunk@47940 1a063a9b-81f0-0310-95a4-ce76da25c4cd