[css-overflow][cssom-view][scroll-animations] Terminology around scroll positions

[fantasai]

Following up on #7312 (comment)

We're a bit handwavy about terminology around scroll positions and scroll offsets. There's the CSSOM scrollTop and scrollLeft interfaces, which imply a particular coordinate model; but we don't have a way to talk about that coordinate model in a more general way.

[fantasai]

Landed a bunch of terminology definitions in https://drafts.csswg.org/css-overflow-3/#scrolling

Leaving this issue open because I'm not sure what to do about the OM, and that update doesn't address the question of if we have a coordinate system (or what it is).