Support Gyp-based projects directly

[kraenhansen]

Instead of relying on the transformation of binding.gyp to CMakeLists.txt (the gyp-to-cmake tool we've built), we could call into the Gyp build tooling directly, effectively expanding our offering to provide a rn-gyp tool wrapping gyp to build for React Native, the same way node-gyp does for Node.js.

[shirakaba]

When originally exploring how to build an .xcframework containing a Node-API module built for multiple triplets, I was unable to see any way to do it using just node-gyp with binding.gyp.

There might be some way to do it by changing the implicit package.json install script to one that calls a more elaborate build script, but I wasn't even able to get as far as making a .framework for any triple, let alone making all triplets and bundling them into an .xcframework.

So by all means, I think it would be great to document how to do it, but I'm not clear how it can be done in the first place!