Convert to monorepo (multi-package repo)

Why?

1. I could move koa-better-body for integration testing. It has extensive test suite and can be great testing for what is happening when changes comes.
2. Also, plugins/parsers can be separate packages, and with separate tests
3. Invite other third party parsers/plugins in future
4. Better code quality and structure

• try yarn v2 eventually

Joining Tidelift network

Due to joining and aligning with Tidelift agreements, we need to enforce 2FA on NPM (and in GitHub). Plus security & other policies, code of conduct (contributor covenant 1.4) ✔️ and etc.

Currently there are too many that have rights on the npm package and I guess it's fair to remove them and make it with 2FA requirement - me, @felixge and @GrosSacASac (and probably one more for fallback if something happens to me, probably @kornelski). Others are welcome to be active again and can be discussed further. @xarguments and @kornelski done much job too. Kornel is an owner in the org, Xargs (and others) doesn't accepting the invitation for a long time. I guess we can close #412 too.

I don't want to take full control or credits. It's just the situation now that it seems I'm the lead recently and in forseeable future. Just want to move forward.

I'm excited for v2 which will include all the things and bugfixes from the tons of previously opened issues and PRs, plus new Streams (#531 by @GrosSacASac) and Plugins API (#545 by me).

Currently we have 2M downloads a week, but I'm not yet sure from where they come. I'll investigate more on that topic and will be able to come up with some plan for the v1 roadmap, future support, security fixes (probably?) and more.

/cc @felixge @charmander @xarguments @quantumsheep @GrosSacASac @kornelski

edit(Feb 12, 2020): removed the npm rights for all non-active people, except @kornelski, @felixge and me.