[css-values] Friendlier `attr()` type syntax (at least for plain numbers)

[Crissov]

I've recently tried to use the new attr() syntax as settled on in #11035.

I very much despise having nested parentheses with type(), especially since it very often just contains a single type in angle brackets, e.g. attr(colspan type(<integer>), 1).

Didn't the old drafts use a keyword as? That would still read a lot nicer in such basic cases: attr(colspan as <integer>, 1). Can we still bring that back?
Alternatively, can we add special-casing for unit-less numbers, i.e. number (and perhaps integer) besides string and the units? attr(colspan number, 1)