Add API for working with Promises

[koistya]

How hard would it be to implement APIs allowing writing Promise-based N-API addons?

[jasongin]

A C API for promises was just recently implemented at nodejs/node#14365

Now it definitely makes sense to expose that in the C++ wrapper. @gabrielschulhof are you planning to do that? Otherwise I might get to it eventually. Or someone else interested may pick it up.

[JamesMGreene]

This issue should be closed now.

[koistya]

@JamesMGreene It would be great to have some docs/examples (/doc/promises.md)

[NickNaso]

@koistya We are working on documentation. What do you think if we add the example or the references to the examples inside the documentation?

[NickNaso]

Hi @koistya,
now we documented the promise api https://github.com/nodejs/node-addon-api/blob/master/doc/promises.md
What do you think should we close this issue?

[NickNaso]

I'm closing the issue because now the API to work with Promise has been added and documented.