LibWeb/CSS: Implement "Create a sum value"

Author: https://github.com/AtkinsSJ
Commit: 80abffd2e8
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/6162
Reviewed-by: https://github.com/gmta ✅

Libraries/LibWeb/CSS/CSSUnitValue.h

@@ -32,6 +32,7 @@ public:
     GC::Ptr<CSSUnitValue> converted_to_unit(FlyString const& unit) const;
 
     virtual bool is_equal_numeric_value(GC::Ref<CSSNumericValue> other) const override;
+    virtual Optional<SumValue> create_a_sum_value() const override;
 
 private:
     explicit CSSUnitValue(JS::Realm&, double value, FlyString unit, NumericType type);