feat: add optional "finally" callback after promise completion

[fellypsantos]

The objective of this simple code increment is to allow execute some code ONLY after promise.toast completes its execution.

In my scenario it's useful to control UI elements that I want to keep disabled while the promise is running, disabling a button to be more specific, and reenabling the button with the "finally" callback, no matter if the promise resolves or rejects.

Example code:

setIsCreatingTodo(true); // one UI button is disabled while todo is being created

toast.promise(createTodo()), {
  loading: 'Loading...',
  success: (createdTodo) => {
    gotoTodoViewer(createdTodo);
    return 'success';
  },
  error: err => {
    const error = err as Error;
    return error.message;
  },
  finally: () => {
    setIsCreatingTodo(false);  // the button can be reactivated 
  }
});