[Request] Add hover effect for toast-close class for better visibility on desktop

Hello.
First of all I have to say that this is a great library. I am using it on an ESP32 and it works flawlessly.

What do you think about adding a hover effect for `toast-close` (the X button) for a better visibility on Desktop.
I am currently using this in my implementation:
```
.toast-close:hover {
    opacity: 1;
}
```

Thank you.
Have a nice day.