Add visualizer for std::flat_map

[Nerixyz]

This PR adds visualizers for std::flat_map with any combination of std::vector and std::deque as the backing containers.

I'm not sure if there's a way of making this smaller. The only difference between the types is the implementation of size, key_at, and value_at - <Expand> is always the same. Ideally, there would be a way of having a condition based on the relevant type in from the template ($T4 for size and key_at; $T5 for value_at). Alternatively, it would be fine keeping just the type where both containers are vectors, since that's the default, and have the fallback that shows keys and values.

Initially, I had a <DisplayString>{{ size={size()} }}</DisplayString>, but I wanted to have at least some preview of the items contained in the map in the unexpanded view. Unfortunately, the [comparator] still shows up in the display string, so only one value is shown.

Code

#include <deque>
#include <flat_map>
#include <string>

template <typename KeyContainer, typename MappedContainer> auto make() {
  std::flat_map<int, std::string, std::less<int>, KeyContainer, MappedContainer>
      map;
  map.emplace(42, "hello world");
  map.emplace(1, "meow");
  map.emplace(2, "hey");
  return map;
}

int main() {
  auto vectorVector = make<std::vector<int>, std::vector<std::string>>();
  auto vectorDeque = make<std::vector<int>, std::deque<std::string>>();
  auto dequeVector = make<std::deque<int>, std::vector<std::string>>();
  auto dequeDeque = make<std::deque<int>, std::deque<std::string>>();
  return 0;
}