An event like `onpop` which gets called when a toast is shown

[rbozan]

I'd like to play an audio fragment if a new toast gets shown, and while I could add a sound effect before I call toast.push or make a wrapper function, can't we have an event in <SvelteToast> like <SvelteToast on:create which gets called when a new toast is pushed? It makes it a bit cleaner and is trivial to add.