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.

CMakeLists.txt

@@ -250,7 +250,7 @@ endif()
 target_precompile_headers(openttd_lib
     PRIVATE
     src/stdafx.h
-    src/3rdparty/fmt/format.h
+    src/core/format.hpp
 )
 
 add_subdirectory(${CMAKE_SOURCE_DIR}/bin)