LibWeb: Avoid dynamic_cast in DOM::Document::is_decoded_svg()

Author: https://github.com/awesomekling
Commit: bbef0e8375
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4395

Libraries/LibWeb/Page/Page.h

@@ -408,6 +408,8 @@ public:
 
     virtual bool is_headless() const = 0;
 
+    virtual bool is_svg_page_client() const { return false; }
+
 protected:
     virtual ~PageClient() = default;
 };