LibJS+LibWeb: Add fast_is<T> helpers for HTML::Script class family

Author: https://github.com/awesomekling
Commit: a9e415b3c4
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4395

Libraries/LibWeb/HTML/Scripting/Script.h

@@ -42,6 +42,7 @@ protected:
     virtual void visit_edges(Visitor&) override;
 
 private:
+    virtual bool is_script() const final { return true; }
     virtual void visit_host_defined_self(JS::Cell::Visitor&) override;
 
     Optional<URL::URL> m_base_url;
@@ -56,3 +57,6 @@ private:
 };
 
 }
+
+template<>
+inline bool JS::Script::HostDefined::fast_is<Web::HTML::Script>() const { return is_script(); }