[feat] Support koa v3.x

[cduff]

Describe the feature

koa v3.0.0 was recently released. @koa/bodyparser can't currently be used with it as it specifies koa as a peer dependency like:

  "peerDependencies": {
    "koa": "^2.14.1"
  },

Can @koa/bodyparser be enhanced to support koa v3.x?

Checklist

• I have searched through GitHub issues for similar issues.
• I have completely read through the README and documentation.