Add `'submenu_file'` filter for the file of an admin menu sub-menu item.

This complements the `'parent_file'` filter added in [12712].

Props swissspidy, kraftbj.
Fixes #24531.

git-svn-id: https://develop.svn.wordpress.org/trunk@34722 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/menu-header.php

@@ -36,6 +36,16 @@ global $menu, $submenu, $parent_file;
  */
 $parent_file = apply_filters( 'parent_file', $parent_file );
 
+/**
+ * Filter the file of an admin menu sub-menu item.
+ *
+ * @since 4.4.0
+ *
+ * @param string $submenu_file The submenu file.
+ * @param string $parent_file  The submenu item's parent file.
+ */
+$submenu_file = apply_filters( 'submenu_file', $submenu_file, $parent_file );
+
 get_admin_page_parent();
 
 /**