Use an option for setting mkdirP's max loop size

[jclem]

• No need to dirty up the environment
• Use a jest pattern for expecting the right error/assertion count

[damccorm]

Is loopMax an option we plan for people to ever use?

If yes, we should probably increase the default to something higher so that configuring it is wholly unnecessary.

If no, it seems weird to me to expose it in the public facing API. I think I'd rather have it as a env variable. If we think that's an issue, I think we should move the implementation into io-helper and still only expose mkdirP(fsPath: string): Promise<void> in io.ts. So that function would look something like:

export async function mkdirP(fsPath: string): Promise<void> {
   return io-util.mkdirP(fsPath, 1000);
}

And we can still test that functionality by directly calling the io-util implementation.

That way nobody takes a dependency on and/or gets confused by loopMax which seems like an implementation detail to me.

[jclem]

Good point, I'll update.

[jclem]

Closed in favor of #9