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

[StephanTLavavej]

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.

[vmichal]

I have fixed problems listed you have listed Stephan T. Lavavej (@StephanTLavavej), but a new strange thing appeared: The visualization of vector iterators and deque iterators is not the same - deque::iterator is visualized as a "normal type" whereas vector::iterator is visualized like a smart pointer, so its visualization contains braces. Check the attached screenshot visualizing flat_multimap<int, int, *,*,*>::iterator for various underlying containers.

Is this desirable?

[StephanTLavavej]

Those braces are definitely weird and confusing.

[vmichal]

Unfortunately, those braces seem consistent with the visualization of std::span::iterator and std::array and possibly more.
A larger change was required, so I separated it to PR #6019 since it is not tightly related to flat_map. However I have tested both changes together and the visualization of flat_(multi)map works as expected.