Add "Manage Patterns" menu item under Pages

snippet.php

<?php
/**
 * Adds a menu item to the admin menu for managing patterns
 */
function s9_add_manage_patterns_menu() {
    // Check if the current user has the capability to edit pages
    if (!current_user_can('edit_pages')) {
        return;
    }

    // Use the add_submenu_page function to add the menu item under Appearance
   add_submenu_page(
        'edit.php?post_type=page', // The slug name for the parent menu (Pages)
        'Manage Patterns', // The text to be displayed in the title tags of the page when the menu is selected
        'Manage Patterns', // The text to be used for the menu
        'edit_pages', // The capability required for this menu to be displayed to the user
        'edit.php?post_type=wp_block' // The slug name to refer to this menu by; should be unique for this menu
    );
}
add_action('admin_menu', 's9_add_manage_patterns_menu');