Get Readonly Keys 
by Anthony Fu @antfu
-gray)
객체의 readonly key 유니언을 반환하는 `GetReadonlyKeys` 제네릭을 구현하세요.
예시
```ts
interface Todo {
readonly title: string
readonly description: string
completed: boolean
}
type Keys = GetReadonlyKeys // expected to be "title" | "description"
```
