STL.natvis: add layout_stride::mapping visualizer

[JMazurkiewicz]

Before:

After:

before-after code

#include <mdspan>

using namespace std;

int main() {
    using E1 = extents<int, 3>;
    layout_stride::mapping<E1> m1({}, array{ 2 });
    (void)m1.stride(0);

    using E2 = dextents<short, 2>;
    layout_stride::mapping<E2> m2(E2{ 3, 4 }, to_array<short>({ 5, 1 }));
    (void)m2.stride(1);

    using E3 = extents<size_t, 4, dynamic_extent, 6>;
    layout_stride::mapping<E3> m3(E3{ 5 }, to_array<size_t>({ 5, 1, 22 }));
    (void)m3.stride(2);
}

Also fixes indentation in extents's visualizer (added in #6053).