Preserve scroll position on browser back

[staydecent]

Router will need to keep a record of scroll positions for each route. So, right before route change, save current scroll position for the current route.

Should be an opt-in feature, as performance may be a concern.

[staydecent]

Looks like this is already handled in Chromium. May be better to await this feature to hit all browsers, then implement a solution ourselves.