This commit introduces the `/wp/v2/menus`, `/wp/v2/menu-items` and `/wp/v2/menu-locations` REST API endpoints. These endpoints are fully available to users with the `edit_theme_options` capability, but can be read by any user who can edit a REST API available post type.
The `nav_menu` taxonomy and `nav_menu_item` post type now map their capabilities to the `edit_theme_options` primitive capability. This allows developers to provide more fine-grained access control. However, if a developer is currently dynamically removing the `edit_theme_options` capability using `map_meta_cap`, they should use the `user_has_cap` filter instead.
The `wp_update_nav_menu_item()` function has been adjusted to return an error if saving the menu item post or assigning the menu item to a menu generate an error.
Lastly, a new menu item type is introduced, `block`, that can be used to store a Block as a menu item.
Props andraganescu, antonvlasenko, dingo_d, dlh, isabel_brison, kadamwhite, Mamaduka, NateWr, noisysocks, peterwilsoncc, ryelle, schlessera, soean, Spacedmonkey, talldanwp, TimothyBlynJacobs, tobifjellner, westonruter, wpscholar, zieladam.
Fixes#40878.
Built from https://develop.svn.wordpress.org/trunk@52079
git-svn-id: http://core.svn.wordpress.org/trunk@51671 1a063a9b-81f0-0310-95a4-ce76da25c4cd
