`STL.natvis`: Need visualizers for `flat_multimap` and `flat_multiset`

The feature branch is currently missing visualizers for `flat_multimap` and `flat_multiset`. We might be able to use "alternative type", or additional work might be required.

Also, I am not super fond of the special cases that have accumulated for `deque`. I consider this to be unusual to the point of not special-casing; `flat_meow` defaults to using `vector` which is way better.