Readonly 
by Anthony Fu @antfu
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Implement the built-in `Readonly` generic without using it.
Constructs a type with all properties of T set to readonly, meaning the properties of the constructed type cannot be reassigned.
For example:
```ts
interface Todo {
title: string
description: string
}
const todo: MyReadonly = {
title: "Hey",
description: "foobar"
}
todo.title = "Hello" // Error: cannot reassign a readonly property
todo.description = "barFoo" // Error: cannot reassign a readonly property
```
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