feat: add option to close overlapped toasts

[jisu15-kim]

Hello ! @BastiaanJansen
When multiple toast messages appear, the existing messages remain visible.
I suggest an option that naturally dismisses the existing messages when new toast messages overlap.

here is an example

let config = ToastConfiguration(allowToastOverlap: false)
let toast = Toast.default(
       image: UIImage(systemName: "airpodspro")!,
       title: "Airpods Pro",
       subtitle: "Connected",
       config: config
)
toast.show()