Support fmt and std::format in C++20 module (#394)

Author: CrackedMatter

module/magic_enum.cppm

@@ -1,7 +1,13 @@
 module;
 
 #include <version>
+#if __has_include(<fmt/format.h>)
+#include <fmt/format.h>
+#endif
 #ifndef MAGIC_ENUM_USE_STD_MODULE
+#if defined(__cpp_lib_format)
+#include <format>
+#endif
 #include <magic_enum/magic_enum_all.hpp>
 #endif
 
@@ -85,3 +91,9 @@ export namespace std {
     using std::formatter;
 }
 #endif
+
+#if defined(FMT_VERSION)
+export namespace fmt {
+    using fmt::formatter;
+}
+#endif