LibJS: Implement parsing and execution of optional chains

Author: https://github.com/alimpfard
Commit: 72ddaa31e3
Pull-request: https://github.com/SerenityOS/serenity/pull/10026
Reviewed-by: https://github.com/davidot
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibJS/Runtime/Reference.h

@@ -97,6 +97,12 @@ public:
         return !m_this_value.is_empty();
     }
 
+    // Note: Non-standard helper.
+    bool is_environment_reference() const
+    {
+        return m_base_type == BaseType::Environment;
+    }
+
     void put_value(GlobalObject&, Value);
     Value get_value(GlobalObject&, bool throw_if_undefined = true) const;
     bool delete_(GlobalObject&);