Add proper C++20 module support (#4799)

src/modules/json.cppm

@@ -0,0 +1,22 @@
+module;
+
+#include <nlohmann/json.hpp>
+
+export module nlohmann.json;
+
+export namespace nlohmann {
+    template <typename T = void, typename SFINAE = void>
+    using adl_serializer = ::nlohmann::adl_serializer<T, SFINAE>;
+
+    using basic_json = ::nlohmann::basic_json<>;
+
+    using json = ::nlohmann::json;
+
+    template <typename RefStringType>
+    using json_pointer = ::nlohmann::json_pointer<RefStringType>;
+
+    using ::nlohmann::ordered_json;
+
+    template <class Key, class T, class IgnoredLess, class Allocator>
+    using ordered_map = ::nlohmann::ordered_map<Key, T, IgnoredLess, Allocator>;
+}  // namespace nlohmann