[css-values-5] Make `type()` optional in `attr()`

[valtlai]

Spec: https://drafts.csswg.org/css-values-5/#attr-notation

Could we allow omitting the type() function in attr() when the attribute type can be described by a single <syntax-component>? Then we could write attr(max <number>), for example.

This would be consistent with the @function parameter and return types (https://drafts.csswg.org/css-mixins/#function-rule).