Fix global UDLs (#3646)

* Add ci_test_noglobaludls to CI

* Really default JSON_GLOBAL_UDLS to 1

* Test global UDLs

* Suppress warnings

* Clarify documentation

tests/src/unit-element_access2.cpp

@@ -10,7 +10,9 @@
 #include "doctest_compatibility.h"
 
 #include <nlohmann/json.hpp>
-using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#ifdef JSON_TEST_NO_GLOBAL_UDLS
+    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
+#endif
 
 TEST_CASE_TEMPLATE("element access 2", Json, nlohmann::json, nlohmann::ordered_json)
 {