add_action( 'rest_api_init', 'custom_cart_endpoint' );
  
function custom_cart_endpoint() {

    register_rest_route(
        'wc-api/my-custom-cart/v1', 
        '/cart',
        [
            'methods' => 'GET',
            'callback' => 'get_cart_data',
        ]   
    );

}
  
function get_cart_data() {



}