Factor out N-API to expand embeddability use cases

[digitalinfinity]

As @bghgary mentioned on the N-API WG meeting on 10/18, there is a use case where developers who are embedding JS engines directly (and not embedding Node) can need a VM-neutral interface to talk to these engines in an ABI-stable manner. Given that N-API already has this, @bghgary suggested factoring out the ES-specific parts of N-API into its own header. Opening this issue to continue that discussion.

@bghgary's prototype is here: https://github.com/bghgary/node-addon-api/tree/ES-only

[mhdawson]

I think its a great suggestion. Making it easier for other projects/teams to consume N-API should help overall adoption. In addition it will increase the chances of these projects contributing back enhancements.

[bghgary]

See nodejs/node#23786

[gabrielschulhof]

Landed in nodejs/node@596bd5f.