Script Loader: Use a global variable in wp_script_modules().

This brings the function more in line with its related `wp_scripts()` and `wp_styles()` functions and makes it easier to reset the class instance in tests.

Props westonruter, luisherranz.
See #56313.
Built from https://develop.svn.wordpress.org/trunk@57503


git-svn-id: http://core.svn.wordpress.org/trunk@57004 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-settings.php

@@ -378,6 +378,8 @@ require ABSPATH . WPINC . '/class-wp-script-modules.php';
 require ABSPATH . WPINC . '/script-modules.php';
 require ABSPATH . WPINC . '/interactivity-api.php';
 
+wp_script_modules()->add_hooks();
+
 $GLOBALS['wp_embed'] = new WP_Embed();
 
 /**