ts：索引类型keyof
https://www.tslang.cn/docs/handbook/advanced-types.html例子1这三个 keyof 效果相同，直接生成一个联合属性 name | ageclass KeyCls0 { name: string; age: number;};interface KeyCls1 { name: string; age: number;};type KeyCls2 = { name: string, age: number};f
