Use more succinct names for start/end

[majido]

This is a bit of bikeshedding but as I have been building demos based on current API I find that some of the property names used for scroll timeline options are unnecessary verbose. So I suggest the following changes:

• startScrollOffset -> start
• endScrollOffset -> end
• scrollSource -> source (optional)

This first two are particularly important once we also allow element-based start and end concepts (as proposed in https://github.com/WICG/scroll-animations/issues/51) in addition to vanilla scroll offsets:

That will give you the following IDL

dictionary ScrollTimelineOptions {
  Element? source = null;
  ScrollDirection orientation = "block";
  DOMString start = "auto";
  DOMString end = "auto";
  (double or ScrollTimelineAutoKeyword) timeRange = "auto";
  FillMode fill = "none";
};

[majido]

Specification has been updated. Marking this as fixed.