获取只读属性 
by Anthony Fu @antfu
-gray)
实现泛型`GetReadonlyKeys`，`GetReadonlyKeys`返回由对象 T 所有只读属性的键组成的联合类型。
例如
```ts
interface Todo {
readonly title: string
readonly description: string
completed: boolean
}
type Keys = GetReadonlyKeys // expected to be "title" | "description"
```
