Add interpolate-size: allow-keywords

[atjn]

The interpolate-size property enables animations to and from keywords like auto, which was previously not possible. Implementers wanted to add this behavior by default, but could not because too many websites rely on the old, limited, behavior.

I don't think we should add it yet, as it is a draft and only supported by Chromium browsers, but I think we should track it and possibly add it in the future if more browsers decide to support it.

Spec: https://www.w3.org/TR/css-values-5/#interpolate-size
Explainer: https://github.com/w3c/csswg-drafts/blob/main/css-values-5/calc-size-explainer.md