New wp_die_app_handler context in wp_die() for APP requests. Introduces...

New wp_die_app_handler context in wp_die() for APP requests. Introduces _scalar_wp_die_handler() as a generic handler that wraps die(), for use by plugins. Move deprecated function to the end of the wp-app.php file (same as xmlrpc.php). see #20042.

git-svn-id: https://core.svn.wordpress.org/trunk@20063 1a063a9b-81f0-0310-95a4-ce76da25c4cd