STL.natvis: add extents visualizer

[JMazurkiewicz]

Before:

After:

before-after code

#include <mdspan>

using namespace std;

int main() {
  extents<signed char> e0;

  extents<unsigned short, 2, 3> e1;
  (void)e1.extent(0);
  (void)e1.static_extent(0);
  
  dextents<int, 2> e2(5, 7);
  (void)e2.extent(0);
  (void)e2.static_extent(0);

  extents<unsigned long, dynamic_extent, dynamic_extent, 17> e3(11, 13);
  (void)e3.extent(0);
  (void)e3.static_extent(0);

  extents<long long, 19, 23, dynamic_extent, 31> e4(29);
  (void)e4.extent(0);
  (void)e4.static_extent(0);
}