Move the user-defined string literals to <nlohmann/json_literals.hpp>

Following the review in #5294, the literals now live in their own header
instead of being removed entirely: <nlohmann/json.hpp> includes it at the
end unless JSON_NO_AUTOMATIC_UDLS (renamed from JSON_NO_UDLS) is defined,
so a project can opt out globally and include the header only where the
literals are used.

The header only uses public and standard macros, because the library's
internal macros are undefined at the end of json.hpp and the amalgamation
inlines macro_scope.hpp only once. For the same reason, the library no
longer defines and undefines JSON_USE_GLOBAL_UDLS, so a user's definition
is still visible to the header. The single-header copy is identical to
the multi-header one, as it only includes <nlohmann/json.hpp>. The module
always exports the literals.

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

src/modules/json.cppm

@@ -18,6 +18,7 @@ module;
 // See: https://github.com/nlohmann/json/issues/5103
 
 #include <nlohmann/json.hpp>
+#include <nlohmann/json_literals.hpp>
 
 export module nlohmann.json;
 
@@ -32,7 +33,6 @@ using NLOHMANN_JSON_NAMESPACE::ordered_json;
 using NLOHMANN_JSON_NAMESPACE::ordered_map;
 using NLOHMANN_JSON_NAMESPACE::to_string;
 
-#ifndef JSON_NO_UDLS
 inline namespace literals
 {
 inline namespace json_literals
@@ -41,7 +41,6 @@ inline namespace json_literals
     using NLOHMANN_JSON_NAMESPACE::literals::json_literals::operator""_json_pointer;
 } // namespace json_literals
 } // namespace literals
-#endif
 
 // Note: the following nlohmann::detail symbols must be exported due to
 // an MSVC bug failing to compile without these symbols visible (ticket #3970)