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).
Sure that'd work, will update.

On a related note -- as I started building this file it became clear that it'd be really useful if I could group module-specific types within the `declare module` declarations and choose to (or not to) export them. This is similar to what I mentioned this the other day, but we were in a rush to get things going so there wasn't time to think it through very far.

So more specifically, something like

```
declare module "child_process" {
  type spawnOpts = {
  };
  export function spawn(modulePath: string, ... options?: spawnOpts): child_process$ChildWorker

  ...
}
```

As-is, all the other module declarations in this file have access to that `spawnOpts` type, even though it's unnecessary for them to. As such, I wind up having to namespace things like that carefully.