Make the PromiseValue type handle nested promises

[onichandame]

As per this discussion, a more realistic use case of unwrapping a type from Promise is when the Promise is resolved to another Promise.

For example, await Promise.resolve(Promise.resolve(2)) is resolved as number, not Promise<number>. The current type PromiseValue however, does not reflect this behaviour.

Thanks to Gerrit0 who shared the trick, I stole his brilliant idea and brought to this package.

[sindresorhus]

Shouldn't PromiseValue just do this by default? Instead of a separate type.

[onichandame]

• reason 1: I thought it would be clearer for the reviewer if changes are isolated in new files

• reason 2: Let me quote from the creator of this type, Gerrit0 said,

  You can work around this issue by using a recursive type alias, but be aware that this is officially not supported and may break in future versions.
  

  Therefore I think it would be better to keep a fallback type PromiseValue so that when this type is broken someday, the maintainer can simply delete this type or alias it to PromiseValue to workaround the possible fatal bugs.

• reason 3: There may be some use cases where people do not want to recursively unwrap the promise(although I cannot think of any)

So I decided to post the changes first and see how people think about this.

[sindresorhus]

Are you aware that they plan to solve this problem in TypeScript itself? microsoft/TypeScript#27711 (comment)

[sindresorhus]

I doubt it will break in the future, but if so, we could probably find a workaround.

[onichandame]

I did look at that issue. The problem is that the official support won't be available before TS 4.0. Before TS people have solved the problem, it is necessary to have this type as a temporary workaround.

I do wish to alter the behaviour of PromiseValue so that it unwraps recursively. So if you agree too, I will make another commit that implements it.