Add RequiredKeys and OptionalKeys types

[saitonakamura]

I've added RequiredKeys and OptionalKeys helpers that extract keys that either required or optional accordingly. It can be useful for example to produce a Record that saves optionality

type Foo = {
  x: string,
  a: number,
  y?: number,
  b?: string,
}

type RecordOptional<K extends keyof any, T> = {
  [P in K]?: T;
};

// Not a record to be fair, cause it accepts type instead of keyof
// but I haven't figured out a good name
type RecordWithSavingOptional<T1, T2> =
  Record<RequiredKeys<T1>, T2>
  & RecordOptional<OptionalKeys<T1>, T2>

// $ExpectError "a" is missing
const bar: RecordWithSavingOptional<A, Date> = { x: new Date() }

It also works only with known keys, because of the infer magic. I tried to use the same approach (adding [keyof T] to the end) as in KeysOfType to no avail, it gives me "a" | "b" | undefined for some reason.

Idea is taken from microsoft/TypeScript#12215 (comment) so kudos to @jcalz @ferdaber and @ajafff!

[saitonakamura]

I'm gonna also add it to readme, but i'm not getting why dtslint formatting is complaining, i haven't even changed some of parts he's complaining about. And it doesn't complain locally. I believe the fact that i'm on windows to be the culprit, though i'm not sure how to fix it

[gcanti]

@saitonakamura nice, thanks!

[stefanobaghino]

This is very nice, thanks for contributing this @saitonakamura! Today I had a few issues trying to define a generic type with all the required keys in another, like in the following example:

type RequiredPart<A extends object> = Pick<A, RequiredKeys<A>>

I spotted what could have lead to the issue and hopefully solved it, I'd love to have your opinion on my PR.