[now-node] Add NowApiHandler type

[mcansh]

now equivalent of NextApiHandler introduced vercel/next.js#10573

[styfle]

Can you create a new file test/fixtures/15-helpers/ts/handler.ts to test this new behavior?

[mcansh]

would that be as simple as this?

import { NowApiHandler } from './types';

const listener: NowApiHandler = (req, res) => {
  res.status(200);
  res.send('hello:RANDOMNESS_PLACEHOLDER');
};

export default listener;

[styfle]

Thanks!