Add SetNullable type

[derodero24]

Closes #697

Adds SetNullable<BaseType, Keys> — the inverse of SetNonNullable. Makes specified keys nullable by adding | null to their value types, while preserving readonly and optional modifiers.

If no keys are specified, all keys are made nullable (matching SetNonNullable's default behavior).

Implementation mirrors SetNonNullable using a homomorphic mapped type.

[som-sm]

PR LGTM, just one tiny question.

[som-sm]

WDYM by this? What's failing here?

[som-sm]

Also, maybe we should remove the keyof constraint from the second argument because, in the case of unions, it prevents updating keys that are not common across all members.

Currently, the JSDoc says:

If no keys are given, all keys will be made nullable.

But this is not true for unions:

type T = SetNullable<{a: number; b: number} | {a: string; c: string}>;
//=> {a: number | null; b: number} | {a: string | null; c: string}

And it's not that this behavior cannot be achieved. It can be achieved by using any as the second argument:

type T = SetNullable<{a: number; b: number} | {a: string; c: string}, any>;
//=> {a: number | null; b: number | null} | {a: string | null; c: string | null}

So even if we keep the keyof constraint, I guess a better default would be any. But I think the constraint should either be removed completely or at least replaced with KeysOfUnion. @sindresorhus WDYT?

[sindresorhus]

I think we should use KeysOfUnion.

We should probably make the same change to SetNonNullable too for consistency.