Generator function APIs

[gabrielschulhof]

A way to create generator functions with N-API functions. (You can already
fake it, but real generator functions may help a bit better, will work if
you use the actual generator methods rather than others with the same
name, and may be more efficient in some cases maybe.)

Attaching a finalizer callback to the generator object can also help.

[gabrielschulhof]

Re: nodejs/node#14256

[bnoordhuis]

How would that work? Note that V8 won't let you create generator functions from C++ either.

[mhdawson]

I think we can close this. We've not actioned it for a number of years. Closing, please let us know if you think that was not the right thing to do.