LibJS: Fix casting a value to ScriptFunction without checking it's one

Author: https://github.com/bugaevc
Commit: 53a94b8bbd
Pull-request: https://github.com/SerenityOS/serenity/pull/2471

Libraries/LibJS/Runtime/Function.h

@@ -42,6 +42,8 @@ public:
 
     virtual void visit_children(Visitor&) override;
 
+    virtual bool is_script_function() const { return false; }
+
     BoundFunction* bind(Value bound_this_value, Vector<Value> arguments);
 
     Value bound_this() const