Keep the behavior-changing configuration readable after json.hpp

json.hpp undefines JSON_STRICT_NUL_HANDLING and
JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON at its end. Code that builds on
the library after it, such as the planned json_view.hpp, reads them from
detail::abi_config instead. The constants live in the ABI namespace, which
already encodes both settings, so they always match the basic_json in use.

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

include/nlohmann/json.hpp

@@ -43,6 +43,7 @@
 
 #include <nlohmann/adl_serializer.hpp>
 #include <nlohmann/byte_container_with_subtype.hpp>
+#include <nlohmann/detail/abi_config.hpp>
 #include <nlohmann/detail/conversions/from_json.hpp>
 #include <nlohmann/detail/conversions/to_json.hpp>
 #include <nlohmann/detail/exceptions.hpp>