Add std::format and fmt support (#5224)

* ✨ add std::format and fmt support

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* ♻️ reorganize PR

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 💚 fix build

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 💚 fix build

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 💚 fix build

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/macro_scope.hpp

@@ -153,6 +153,14 @@
     #endif
 #endif
 
+#ifndef JSON_HAS_STD_FORMAT
+    #if defined(JSON_HAS_CPP_20) && defined(__cpp_lib_format)
+        #define JSON_HAS_STD_FORMAT 1
+    #else
+        #define JSON_HAS_STD_FORMAT 0
+    #endif
+#endif
+
 #ifndef JSON_HAS_STATIC_RTTI
     #if !defined(_HAS_STATIC_RTTI) || _HAS_STATIC_RTTI != 0
         #define JSON_HAS_STATIC_RTTI 1

include/nlohmann/detail/macro_unscope.hpp

@@ -41,6 +41,7 @@
     #undef JSON_HAS_EXPERIMENTAL_FILESYSTEM
     #undef JSON_HAS_THREE_WAY_COMPARISON
     #undef JSON_HAS_RANGES
+    #undef JSON_HAS_STD_FORMAT
     #undef JSON_HAS_STATIC_RTTI
     #undef JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
 #endif