[css-mixins-1] Review of @macro

[andruud]

In #12927, we resolved to add an unscoped mixin variant:

RESOLVED: Add an unscoped mixin variant (syntax TBD)

This is now specified as @macro, which, at least to me, makes a lot of sense. (In particular, the absence of arguments seems wise). However, since the previous resolution landed at "syntax TBD", we should take an explicit resolution to approve it.

Note: There was some (after-the-fact) discussion around @macro from #12927 (comment) and onwards.