Codechange: fmt (and std::format) do explicitly not support enums out-of-the-box

That it works for the version we have packaged it pure coincidence, as that is
one of the few versions that due to a bug allow it. So add the appropriate
template specialisations to support it out-of-the-box within OpenTTD.

src/debug.h

@@ -12,7 +12,7 @@
 
 #include "cpu.h"
 #include <chrono>
-#include "3rdparty/fmt/format.h"
+#include "core/format.hpp"
 
 /* Debugging messages policy:
  * These should be the severities used for direct Debug() calls