Readonly 2 
by Anthony Fu @antfu
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Implement a generic `MyReadonly2` which takes two type argument `T` and `K`.
`K` specify the set of properties of `T` that should set to Readonly. When `K` is not provided, it should make all properties readonly just like the normal `Readonly`.
For example
```ts
interface Todo {
title: string
description: string
completed: boolean
}
const todo: MyReadonly2 = {
title: "Hey",
description: "foobar",
completed: false,
}
todo.title = "Hello" // Error: cannot reassign a readonly property
todo.description = "barFoo" // Error: cannot reassign a readonly property
todo.completed = true // OK
```
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