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Serverless Next.js (#5927)

**This does not change existing behavior.**

building to serverless is completely opt-in.

- Implements `target: 'serverless'` in `next.config.js`
- Removes `next build --lambdas` (was only available on next@canary so far)

This implements the concept of build targets. Currently there will be 2 build targets:

- server (This is the target that already existed / the default, no changes here)
- serverless (New target aimed at compiling pages to serverless handlers)

The serverless target will output a single file per `page` in the `pages` directory:

- `pages/index.js` => `.next/serverless/index.js`
- `pages/about.js` => `.next/serverless/about.js`

So what is inside `.next/serverless/about.js`? All the code needed to render that specific page. It has the Node.js `http.Server` request handler function signature:

```ts
(req: http.IncomingMessage, res: http.ServerResponse) => void
```

So how do you use it? Generally you **don't** want to use the below example, but for illustration purposes it's shown how the handler is called using a plain `http.Server`:

```js
const http = require('http')
// Note that `.default` is needed because the exported module is an esmodule
const handler = require('./.next/serverless/about.js').default
const server = new http.Server((req, res) => handler(req, res))
server.listen(3000, () => console.log('Listening on http://localhost:3000'))
```

Generally you'll upload this handler function to an external service like [Now v2](https://zeit.co/now-2), the `@now/next` builder will be updated to reflect these changes. This means that it'll be no longer neccesary for `@now/next` to do some of the guesswork in creating smaller handler functions. As Next.js will output the smallest possible serverless handler function automatically.

The function has 0 dependencies so no node_modules are required to run it, and is generally very small. 45Kb zipped is the baseline, but I'm ... (continued)

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