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/core/CMakeLists.txt

@@ -8,6 +8,7 @@ add_files(
     endian_func.hpp
     endian_type.hpp
     enum_type.hpp
+    format.hpp
     geometry_func.cpp
     geometry_func.hpp
     geometry_type.hpp