LibJS: Implement rest parameters

Author: https://github.com/linusg
Commit: 454c1e6bbe
Pull-request: https://github.com/SerenityOS/serenity/pull/2099
Reviewed-by: https://github.com/awesomekling

Libraries/LibJS/AST.h

@@ -158,6 +158,7 @@ public:
     struct Parameter {
         FlyString name;
         RefPtr<Expression> default_value;
+        bool is_rest { false };
     };
 
     const FlyString& name() const { return m_name; }