Move formatter<std::error_code> from fmt/os.h to fmt/std.h (#3125)

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>

test/std-test.cc

@@ -10,6 +10,7 @@
 #include <string>
 #include <vector>
 
+#include "fmt/os.h"  // fmt::system_category
 #include "fmt/ranges.h"
 #include "gtest-extra.h"  // StartsWith
 
@@ -83,6 +84,18 @@ TEST(std_test, variant) {
 #endif
 }
 
+TEST(std_test, error_code) {
+  EXPECT_EQ("generic:42",
+            fmt::format(FMT_STRING("{0}"),
+                        std::error_code(42, std::generic_category())));
+  EXPECT_EQ("system:42",
+            fmt::format(FMT_STRING("{0}"),
+                        std::error_code(42, fmt::system_category())));
+  EXPECT_EQ("system:-42",
+            fmt::format(FMT_STRING("{0}"),
+                        std::error_code(-42, fmt::system_category())));
+}
+
 template <typename Catch> void exception_test() {
   try {
     throw std::runtime_error("Test Exception");