Provide `mdspan` C++26 slices: `range_slice` and `extent_slice` [P3982R2]

### Is this a duplicate?

- [x] I confirmed there appear to be no [duplicate issues](https://github.com/NVIDIA/cccl/issues) for this request and that I agree to the [Code of Conduct](CODE_OF_CONDUCT.md)

### Area

libcu++

### Is your feature request related to a problem? Please describe.

Simplify `submdspan` slicing descriptors

### Describe the solution you'd like

Implement https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2026/p3982r2.html, see https://eel.is/c++draft/views.multidim

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start in the libcu++ mdspan and submdspan implementation, then read the linked P3982R2 paper and the C++ draft section on multidimensional views. Implement the range_slice and extent_slice descriptors as specified there, with submdspan slicing behavior matching the proposal.

Written by the indexing model from the issue text.