Re: [csswg-drafts] [scroll-animations-1] Something is odd with the handling of `source` in the ScrollTimeline constructor (#5202)

I guess the idea is that if the `ScrollTimelineOptions` may have a null source (which is the default value). And if that is given as input we resolve it to root scroller. So constructed timeline will never have a null source.

However thinking about this more, I wonder we should allow a three state in our handling
- Unspecified: Use default value as document scroller e.g., `new ScrollTimeline({source: null})`  
- Explicitly set to null:  set source to to null  e.g., `new ScrollTimeline({source: null}) `
- Explicitly set to element:  set source to element  e.g., `new ScrollTimeline({source: $div}) `


This is similar to how timeline is handler on `Animation` constructor.
I can make this change and clear up the wording mistake as well.


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