[css-scroll-snap][cssom] Define interaction of ScrollIntoView options and scroll-snap

Scroll Snap and CSSOM View currently conflict with each other: they both attempt to define the position of the element wrt the viewport when scrolled into view.

For sure, Scroll Snap should win when there's no arguments given to the API.

I also think that the positioning arguments should be dropped in favor of control with the scroll snap properties, unless there's a compelling reason why a JS API call needs a different value than is given declaratively in CSS. (Note, according to MDN, ScrollIntoViewOptions have only been implemented in Gecko.)