Export the ToastProps type

[gauntface]

From what I can tell, there is no way for a user of this module to get the ToastProps type, so typing can't be achieve for the result of functions like toasts.add().

import {ToastProps, toasts} from "svelte-toasts";

let toast: ToastProps | undefined;
if (!toast) {
    toast = toasts.add(...);
}