[feat] Add a `State` type export

## 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.

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

## Checklist

- [x] I have searched through GitHub issues for similar issues.
- [x] I have completely read through the README and documentation.

Start at the package's public export entry point and inspect how its current middleware types use Koa.DefaultState. Add the requested State type export with the documented JWT payload and original-error properties, then verify that consumers can import it without constructing the interface themselves.

Written by the indexing model from the issue text.