n-api: implement promise

Promise is implemented as a pair of objects. `napi_create_promise()`
returns both a JavaScript promise and a newly allocated "deferred" in
its out-params. The deferred is linked to the promise such that the
deferred can be passed to `napi_resolve_deferred()` or
`napi_reject_deferred()` to reject/resolve the promise.

`napi_is_promise()` can be used to check if a `napi_value` is a native
promise - that is, a promise created by the underlying engine, rather
than a pure JS implementation of a promise.

PR-URL: https://github.com/nodejs/node/pull/14365
Fixes: https://github.com/nodejs/abi-stable-node/issues/242


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>