Support a promise API

[SimenB]

(Sorta surprised no-one has opened an issue about this, sorry if one exists and my searching is just bad (there is sorta #151, but this issue is about native promise support, not promisification)).

Node's new ESM resolution supports promises, but not callbacks. If we wanna use resolve to implement a custom linker, a custom loader hook or the recently landed import.meta.resolve we'd need to manually wrap resolve in a promise - it would be lovely if resolve natively worked with promises out of the box. I realise these APIs are behind experimental flags (or not even in any release), but I don't think there's any reason to believe the functionality these APIs provide will not be available whenever the APIs stabilize, and that they won't be based on promises.

While one can promisify resolve manually, the problem is a bit extrapolated when considering the pluggable FS operations (recent versions of Node also ship with a promise based FS API). Based on #151 util.promisify doesn't work out of the box either.

If accepted, I'm happy to work on the implementation.

[Cheesetouched]

Sounds good @SimenB! Although, what exactly are you suggesting? Converting existing methods or adding another one?

[SimenB]

#224 (comment) seems most likely (so a new API)