[scroll-animations-1] Use of DOMString in API surface

[plinss]

Small point that came up in the TAG Review, the ScrollTimeline interface is currently using DOMString to represent keyword and length/percentage values. This should use the appropriate CSS Typed OM classes instead.

Using strings for CSS values is a historical artifact we're trying to eliminate, new APIs shouldn't use them.

[majido]

Thanks. This has been something that was suggested before as well by @andruud in #4352 as well.
Given TAG's recommendation here we should go ahead and make this change. I also like Tab's suggestion to treat strings implicitly as keyword.