Add new programmatic usage

[timneutkens]

Related: vercel/next.js#7297

For serverless functions, @now/node:

const micro = require('micro')

module.exports = micro((req, res) ⇒ {
   return 'Hello World'
})

Current invoking micro() will return a Node.js http.Server which doesn't work well with just req, res as a handler. This also ties into using micro standalone with @now/node. Doing this will also allow us to deprecate micro-dev too in favor of using Next.js / now dev + @now/node

The new programmatic usage works everywhere and really makes Micro a layer on top of the default Node.js request handling.

This solves:

• Easier usage (no extra steps needed besides installing and using, like setting up the commands in package.json etc is no longer needed)
• Easier deployment
• Scalable by default
• Serverless by default

[javivelasco]

I think it could be a good idea to add the index.d.ts file with proper types so we can stop using @types/micro and there will be no need to update there either.

[timneutkens]

Sounds great to me, but out of scope for this PR 👍

[zartdinov]

Looks like need to update README.md.

https://github.com/zeit/micro#programmatic-use
https://github.com/zeit/micro#testing

I had some problems with using http.Server

[timneutkens]

Actually no, the readme is correct. It's just not released on stable yet. You can check https://npmjs.com/micro for the stable readme.

[zartdinov]

I understood, thanks!