Add a new type PublicOf

Suggestion

Add the PublicOf type suggested in this issue's comment by @DanielRosenwasser

Use Cases

Implement a class public properties/methods without the need to implement private/protected ones.

Examples

// type to add
type PublicOf<T> = {
    [P in keyof T]: T[P]
}

// use case
class C {
    private foo();

    public bar();
}

// No need to implement private method foo !
class D implements PublicOf<C> {
    public bar() {}
}

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Start by reviewing the linked suggestion and the examples in this issue, then examine how TypeScript currently handles mapped types, keyof, and implements checks. Done should include a supported PublicOf type whose use allows class D to implement C's public members without implementing private or protected members, with appropriate type-checking coverage.