SetOptional/SetRequired/SetReadonly: Fix when the second argument is any

[som-sm]

Currently, when the second argument to Set-Optional is any, it returns an index signature. And this because Set-Optional uses Pick.

type T = SetOptional<{a: 1; b: 2}, any>; // => type T = {[x: string]: any}

This PR modifies this behaviour to instead return all keys marked as optional.

type T = SetOptional<{a: 1; b: 2}, any>; // => type T = {a?: 1; b?: 2}

Similar changes made for Set-Required & Set-Readonly.

Additionally, also fixes Set-Optional when instantiated with unions.

[sindresorhus]

@som-sm Thanks for all the recent good contributions 🙏