Start a node stdlib interface file

This starts the beginnings of an interface file for node built-in libraries.

I've filled in a couple of the libraries as of right now, but I've also setup the skeleton for all the others (so that it's not bottlenecked on me doing all of them, and others can help fill them in as well, piecemeal).
Why not use type ... = { ... } instead of declare class ... { ... } for these? (In general, declare class should be used when we actually want constructor behaviors, statics, and other things that make classes classes.)