Improve NextApiResponse typing

[VincentCordobes]

Introducing two changes here

Fix NextApiResponse.status type signature 989e549

According to the doc we can chain the function call (thanks to here)

Improve NextApiResponse typing fb11fd2

Makes the NextApiResponse accept a type argument
Currently res.json(...) is not type checked but it could be helpful to see at a glance the handler signature, like so:

export type MyResponse =
  | {ok: true; blabla: message}
  | {ok: false; message: string}

export default function handle(
  req: NextApiRequest,
  res: NextApiResponse<MyResponse>,
) {
  //...
  res.json({ok: true, blabla: ...}) // typecheck!!
}

I didn't make the NextApiRequest generic because—we can't trust the request input—we would have to validate it anyway.

What do you think ?

[github-actions]

Stats from current PR

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	zeit/next.js canary	VincentCordobes/next.js canary	Change
Build Duration	14.1s	14.2s	⚠️ +127ms
node_modules Size	45.4 MB	45.4 MB	⚠️ +109 B
Total Bundle (main, webpack, commons) Size	210 kB	210 kB	✓
Total Bundle (main, webpack, commons) gzip Size	68.8 kB	68.8 kB	✓
Client _app Size	2.39 kB	2.39 kB	✓
Client _app gzip Size	1.08 kB	1.08 kB	✓
Client _error Size	8.45 kB	8.45 kB	✓
Client _error gzip Size	3.24 kB	3.24 kB	✓
Client pages/index Size	296 B	296 B	✓
Client pages/index gzip Size	222 B	222 B	✓
Client pages/link Size	4.76 kB	4.76 kB	✓
Client pages/link gzip Size	2.03 kB	2.03 kB	✓
Client pages/routerDirect Size	423 B	423 B	✓
Client pages/routerDirect gzip Size	306 B	306 B	✓
Client pages/withRouter Size	435 B	435 B	✓
Client pages/withRouter gzip Size	301 B	301 B	✓
Client main Size	24.3 kB	24.3 kB	✓
Client main gzip Size	7.97 kB	7.97 kB	✓
Client commons Size	183 kB	183 kB	✓
Client commons gzip Size	59.5 kB	59.5 kB	✓
Client webpack Size	1.49 kB	1.49 kB	✓
Client webpack gzip Size	770 B	770 B	✓
Base Rendered Size	1.36 kB	1.36 kB	✓
Build Dir Size	785 kB	785 kB	✓

Click to expand serverless stats

	zeit/next.js canary	VincentCordobes/next.js canary	Change
Build Duration	16.1s	15.9s	-170ms
node_modules Size	45.4 MB	45.4 MB	⚠️ +109 B
Total Bundle (main, webpack, commons) Size	210 kB	210 kB	✓
Total Bundle (main, webpack, commons) gzip Size	68.8 kB	68.8 kB	✓
Client _app Size	2.39 kB	2.39 kB	✓
Client _app gzip Size	1.08 kB	1.08 kB	✓
Client _error Size	8.45 kB	8.45 kB	✓
Client _error gzip Size	3.24 kB	3.24 kB	✓
Client pages/index Size	296 B	296 B	✓
Client pages/index gzip Size	222 B	222 B	✓
Client pages/link Size	4.76 kB	4.76 kB	✓
Client pages/link gzip Size	2.03 kB	2.03 kB	✓
Client pages/routerDirect Size	423 B	423 B	✓
Client pages/routerDirect gzip Size	306 B	306 B	✓
Client pages/withRouter Size	435 B	435 B	✓
Client pages/withRouter gzip Size	301 B	301 B	✓
Client main Size	24.3 kB	24.3 kB	✓
Client main gzip Size	7.97 kB	7.97 kB	✓
Client commons Size	183 kB	183 kB	✓
Client commons gzip Size	59.5 kB	59.5 kB	✓
Client webpack Size	1.49 kB	1.49 kB	✓
Client webpack gzip Size	770 B	770 B	✓
Serverless pages/link Size	338 kB	338 kB	✓
Serverless pages/link gzip Size	86.7 kB	86.7 kB	⚠️ +1 B
Serverless pages/index Size	Error getting size	Error getting size	✓
Serverless pages/index gzip Size	Error getting size	Error getting size	✓
Serverless pages/_error Size	328 kB	328 kB	✓
Serverless pages/_error gzip Size	84 kB	84 kB	⚠️ +1 B
Serverless pages/routerDirect Size	329 kB	329 kB	✓
Serverless pages/routerDirect gzip Size	84.3 kB	84.3 kB	⚠️ +2 B
Serverless pages/withRouter Size	329 kB	329 kB	✓
Serverless pages/withRouter gzip Size	84.2 kB	84.2 kB	⚠️ +2 B
Build Dir Size	2.04 MB	2.04 MB	✓

[Timer]

This LGTM. Let's wait for @huv1k.