LibWeb/CSS: Implement rectify_a_numberish_value()

Author: https://github.com/AtkinsSJ
Commit: 46b55f0f46
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5955
Reviewed-by: https://github.com/trflynn89

Libraries/LibWeb/CSS/CSSNumericValue.h

@@ -56,4 +56,6 @@ protected:
 // https://drafts.css-houdini.org/css-typed-om-1/#typedefdef-cssnumberish
 using CSSNumberish = Variant<double, GC::Root<CSSNumericValue>>;
 
+GC::Ref<CSSNumericValue> rectify_a_numberish_value(JS::Realm&, CSSNumberish const&, Optional<FlyString> unit = {});
+
 }