add a switch to enable implicit conversions (defaults to true)

wrap implicit conversions tests around the JSON_USE_IMPLICIT_CONVERSIONS
macro

test/src/unit-udt.cpp

@@ -312,6 +312,7 @@ TEST_CASE("basic usage" * doctest::test_suite("udt"))
             CHECK(name.m_val == "new name");
         }
 
+#if JSON_USE_IMPLICIT_CONVERSIONS
         SECTION("implicit conversions")
         {
             const udt::contact_book parsed_book = big_json;
@@ -335,6 +336,7 @@ TEST_CASE("basic usage" * doctest::test_suite("udt"))
             CHECK(book_name == udt::name{"C++"});
             CHECK(book == parsed_book);
         }
+#endif
     }
 }