LibWeb: Implement CSS 'contain' property

Author: https://github.com/Psychpsyo
Commit: 67ed676831
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3296
Reviewed-by: https://github.com/AtkinsSJ ✅

Libraries/LibWeb/CSS/ComputedProperties.h

@@ -161,6 +161,7 @@ public:
     Optional<CSS::WritingMode> writing_mode() const;
     Optional<CSS::UserSelect> user_select() const;
     Optional<CSS::Isolation> isolation() const;
+    CSS::Containment contain() const;
 
     static Vector<CSS::Transformation> transformations_for_style_value(CSSStyleValue const& value);
     Vector<CSS::Transformation> transformations() const;