[scroll-animations] `ViewTimelineOptions.inset` should use `CSSKeywordish` over `CSSKeywordValue`

[graouts]

The inset property of the ViewTimelineOptions is currently defined as (DOMString or sequence<(CSSNumericValue or CSSKeywordValue)>) inset = "auto";. This means that setting that value to "auto" is valid but not setting it to ["auto", "auto"].

I suggest we use CSSKeywordish instead of CSSKeywordValue so that strings can be used as well as CSSKeywordValue. As it turns out, this is already what is being tested in scroll-animations/view-timelines/view-timeline-inset.html and what Chromes implements.

[graouts]

Cc @flackr, @bramus, @kevers-google and @andruud.

[flackr]

I remember from the discussion when we added support for strings the main reason was to allow passing CSS style strings directly for which I imagine you'd pass multiple values space separated: "auto auto". See #7748 (comment)

However, I don't think there's much additional complexity to supporting parsing a string for each argument, so I think making this change is reasonable.