Feature: Split MinMax and ArgMinMax reductions

### Describe what you are looking for

Finding the minimum and the maximum value in a sequence is fundamentally cheaper, than locating that offset position in a vectorized form. It potentially makes sense to split `nk_reduce_minmax` into two APIs:

- `nk_reduce_argminmax` - returning two offsets
- `nk_reduce_minmax` - returning two scalar values

Relates to #315

### Can you contribute to the implementation?

- [x] I can contribute

### Is your feature request specific to a certain interface?

It applies to everything

### Contact Details

_No response_

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Start by locating the existing nk_reduce_minmax API and reviewing related issue #315. Compare how the reduction is exposed across the project's interfaces; done means separate nk_reduce_argminmax offset results from nk_reduce_minmax scalar results everywhere.

Written by the indexing model from the issue text.