fix(struct): `fields` not copy `extra`s correctly

    private _fields: [name: PropertyKey, definition: StructFieldDefinition<any, any, any>][] = [];

    private _extra: PropertyDescriptorMap = {};

    private _extra: Record<PropertyKey, unknown> = {};

    private _postDeserialized?: StructPostDeserialized<any, any> | undefined;

            this._fields.push(field);

        }

        this._size += other._size;

        Object.assign(this._extra, other._extra);

        Object.defineProperties(this._extra, Object.getOwnPropertyDescriptors(other._extra));

        return this as any;

    }