```restoreScrollState``` does not work when page is scrolled only horizontally

[Jan-Kruse]

The scroll state is not restored, when window.scrollX > 0 and window.scrollY = 0. This is caused by the following condition in Router.svelte:

if (event.state && event.state.__svelte_spa_router_scrollY) {
    previousScrollState = event.state
}

It should be changed to something like:

if (event.state && (event.state.__svelte_spa_router_scrollY || event.state.__svelte_spa_router_scrollX)) {
    previousScrollState = event.state
}

[ItalyPaleAle]

Thanks for reporting this :) Would you be interested in opening a PR?

[Jan-Kruse]

@ItalyPaleAle PR #268 is waiting for merge