LibWeb: Add new UnresolvedStyleValue class

This represents a property value that hasn't been converted to a
"proper" StyleValue yet. That is, it's either a custom property's value,
or a value that includes `var()` references, (or both!) since neither of
those can be fully resolved at parse time.

Userland/Libraries/LibWeb/Forward.h

@@ -67,6 +67,7 @@ class StyleValueList;
 class Supports;
 class TextDecorationStyleValue;
 class TransformationStyleValue;
+class UnresolvedStyleValue;
 class UnsetStyleValue;
 }