[RFC]: improve type declarations for `@stdlib/utils/group-in`

### Description

This RFC proposes improving the TypeScript type declarations for `@stdlib/utils/group-in`. Currently, considerable type information is lost when returning results, which are presently typed as `any`.

Ideally, we'd improve declarations similar to [`@stdlib/utils/group-by`](https://github.com/stdlib-js/stdlib/blob/1bf88f1a75366df27e02072018699487680ab597/lib/node_modules/%40stdlib/utils/group-by/docs/types/index.d.ts) which better preserves type info.

### Related Issues

None.

### Questions

No.

### Other

No.

### Checklist

- [X] I have read and understood the [Code of Conduct](https://github.com/stdlib-js/stdlib/blob/develop/CODE_OF_CONDUCT.md).
- [X] Searched for existing issues and pull requests.
- [X] The issue name begins with `RFC:`.