Use `node-api-headers` package instead of full nodejs headers

For modules built using NAN, they need the correct version of the nodejs/electron headers to build against.

For modules built using Node-API, they do not need the full headers and instead only need a small subset. That small subset is available as its own package on npm. https://github.com/nodejs/node-api-headers.

If node-gyp was to use node-api-headers for sourcing the node-api headers, that would provide some benefits:

1. Some builds would be sped up as they wouldnt need to download the full headers. Perhaps node-api-headers could be bundled into node-gyp? Not sure how that would work when new versions are released.

2. Some native modules are accidentally including headers that are not part of node-api and so do not follow the stability guarantees https://nodejs.org/api/n-api.html#implications-of-abi-stability. This is all too easy to do when porting code from NAN to Node-API.

It would be ideal if there was a way for modules to opt into using node-api-headers instead of the full headers, and it should be recommended in the docs for node-api based modules to enable this option.

Perhaps in the next major version it could be looked into enabling this automatically, but I dont know if that would be achievable or if it would break too many modules.

Review the node-api-headers package and node-gyp's current header-sourcing behavior. Clarify whether support should be opt-in or automatic, how it would coexist with full Node.js headers for NAN modules, and what documentation and validation would define completion.