[feat] Add a State type export

[shellscape]

Describe the feature

I'd like to have a State type export so that I don't have to manually construct the state for each app I use this package in. e.g.

export interface State extends Koa.DefaultState {
  // include the [key]: JwtPayload  property
  jwtOriginalError?: Error
}

Checklist

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