LibWeb: Implement CSSStyleDeclaration.cssText

Author: https://github.com/Lubrsi
Commit: c247fefee7
Pull-request: https://github.com/SerenityOS/serenity/pull/15964
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/CSS/ResolvedCSSStyleDeclaration.h

@@ -25,6 +25,7 @@ public:
     virtual WebIDL::ExceptionOr<String> remove_property(PropertyID) override;
 
     virtual String serialized() const override;
+    virtual WebIDL::ExceptionOr<void> set_css_text(StringView) override;
 
 private:
     explicit ResolvedCSSStyleDeclaration(DOM::Element&);