实现 Pick
Description
从类型 T 中选择出属性 K，构造成一个新的类型。
Example
interface Todo { title: string description: string completed: boolean}
type TodoPreview = MyPick<Todo, 'title' | 'completed'>
const todo: TodoPreview = { title: 'Clean room', completed: false,}Answer
type MyPick<T, K extends keyof T> = { [P in K]: T[P] }