LibWeb/CSS: Allow descriptors to be shorthands

I was wrong when I added those notes before about this being impossible,
it's *very* possible, for example with the `@page margin` descriptor.
However, until we have a large number of these shorthands and not just a
single example, we can get away with hard-coding support for it.

Libraries/LibWeb/CSS/CSSDescriptors.h

@@ -45,4 +45,7 @@ private:
     Vector<Descriptor> m_descriptors;
 };
 
+bool is_shorthand(AtRuleID, DescriptorID);
+void for_each_expanded_longhand(AtRuleID, DescriptorID, RefPtr<CSSStyleValue const>, Function<void(DescriptorID, RefPtr<CSSStyleValue const>)>);
+
 }