allow for custom add to cart handlers #5897

this is just one attempt at it #5897

I was thinking about having a switch statement with "simple" as a case and the default being a `do_action('woocommerce_add_to_cart_handler_' . $add_to_cart_handler)`. However, since "simple" is currently the default that *could* mess up anyone else's product type extensions... or at least require extensions to start filtering the `$add_to_cart_handler`.