Add a ReadonlyKeys which finds readonly keys

[voxpelli]

I needed to find out what keys that are assignable and remove all keys but those, but found no way in type-fest or stock typescript, but did stumble upon this trick from @mattmccutchen on Stack Overflow.

Making an issue here to keep track of this, if I get time later then I may wrap up a PR, else feel free to make one for this.

The example from Stack Overflow is:

// https://github.com/Microsoft/TypeScript/issues/27024#issuecomment-421529650
type IfEquals<X, Y, A, B> =
    (<T>() => T extends X ? 1 : 2) extends
    (<T>() => T extends Y ? 1 : 2) ? A : B;

// Alternatively:
/*
type IfEquals<X, Y, A, B> =
    [2] & [0, 1, X] extends [2] & [0, 1, Y] & [0, infer W, unknown]
    ? W extends 1 ? B : A
    : B;
*/

type WritableKeysOf<T> = {
    [P in keyof T]: IfEquals<{ [Q in P]: T[P] }, { -readonly [Q in P]: T[P] }, P, never>
}[keyof T];

[voxpelli]

I did one minor addition when I used this in some code now:

I wrapped the result in NonNullable<>, as else, when at least all keys were optional, a undefined would be appended, and that would make Pick<> refuse it as an input:

export type WritableKeysOf<T> = NonNullable<{
    [P in keyof T]: IfEquals<
      { [Q in P]: T[P] },
      { -readonly [Q in P]: T[P] },
      P,
      never
    >
}[keyof T]>;

[sindresorhus]

👍 PR welcome.