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

[fbusato]

Is this a duplicate?

• I confirmed there appear to be no duplicate issues for this request and that I agree to the Code of Conduct

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