Adds a required `postType` context, which is used in the editor client side. Props bernhard-reiter, cbravobernal. Fixes #63994. Built from https://develop.svn.wordpress.org/trunk@60778 git-svn-id: http://core.svn.wordpress.org/trunk@60114 1a063a9b-81f0-0310-95a4-ce76da25c4cd