fix: only autoplay videos if visible

[themicknugget]

If autoplay is enabled, redlib will start playing all videos on the current page, at the same time.

This introduces an observer to only start autoplaying a video if it's visible, and pause it if it stops being visible. This was my expected behavior when I enabled the "autoplay" setting.

[pimlie]

I guess this should also be fixed by #179 or #181, also added an intersectionobserver in those prs