Svelte: Add a way to customize the conditions of `handleVisibilityChange()`

[Blade67]

As the title suggest, I'd like to be able to override the handleVisibilityChange function to be able to only check the session on page load and not on page visibility change. I would welcome such a change since not all applications need the user to be logged in at all times, especially on SSR pages.

I suggest: Export the handleVisibilityChange function similar to the onUserUpdate function, give it a default function (the current one is fine) and voilà.