[css-fonts-5] make metric value optional for font-size-adjust

[fantasai]

As I was working in the edits for #6160 I noticed that the grammar is a bit more locked down than we'd normally allow, so I'd like us to consider:

• Making the metric value optional (defaulting to from-font), so all that's required is a keyword of which metric we care about. This would make things like font-size-adjust: cap valid (and useful).
• Maybe making the two components reorderable? Unless we plan to extend to allow multiple keyword-value pairs in the future, this would fit better with our usual grammar patterns. This would make things like font-size-adjust: 0.4 cap valid in addition to font-size-adjust: cap 0.4. (I'm less sure about this one.)

[Crissov]

A legal property value like 0.4 cap, that is number + whitespace + keyword-that-looks-like-a-length-unit, looks just wrong, to be frank. The other way, around, cap 0.4, is preferable by a lot.