P2642R6 Padded mdspan Layouts

[StephanTLavavej]

WG21-P2642R6 Padded mdspan Layouts

INCREASED feature-test macro (expected):

#define __cpp_lib_submdspan 202403L

While this increases the feature-test macro for #3807 WG21-P2630R4 submdspan, it seems to be a partially-overlapping feature instead of strictly a patch paper, so I've recorded this separately.

Note: We're focused on implementing the remaining library-only features in C++23. Until that's done, we will NOT be accepting PRs for C++26 features.

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