[feat] koa-qs types for v3.0.0

## Describe the feature

We want to use koa-qs 3.0.0 with the custom decoder function, but the types for koa-qs in `@types/koa-qs` do not exist for version 3. is that something currently in progress? thanks so much

## Checklist

- [x] I have searched through GitHub issues for similar issues.
- [x] I have completely read through the README and documentation.
