Posts: Introduce WP_Post_Type and use it in register_post_type() and unregister_post_type().

This changes the global `$wp_post_types` to an array of `WP_Post_Type` objects. `WP_Post_Type` includes methods to handle post type supports, rewrite rules, meta boxes, hooks, and taxonomies.
Each post type argument becomes a property of `WP_Post_Type`.

Props swissspidy, flixos90.
Fixes #36217.

git-svn-id: https://develop.svn.wordpress.org/trunk@37890 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/edit.php

@@ -23,8 +23,8 @@ if ( 'attachment' === $typenow ) {
 }
 
 /**
- * @global string $post_type
- * @global object $post_type_object
+ * @global string       $post_type
+ * @global WP_Post_Type $post_type_object
  */
 global $post_type, $post_type_object;