Name the exception ids and address review comments

Add detail::exception_id, a scoped enum with one named enumerator per
documented exception id, and use it for every id in the library. The
create() functions get an overload for it; the int overloads stay for
user code.

Following the review of #5783: add binary_reader::invalid_byte() and
length_type_error(), basic_json::throw_subscript_wrong_type(), move the
fuzzer includes and the using-declaration into fuzzer_common.hpp, and
rename test_sax.hpp to sax_event_loggers.hpp.

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

tests/src/unit-class_parser.cpp

@@ -38,7 +38,7 @@ using nlohmann::json;
 #include <vector>
 
 #include "sax_countdown.hpp"
-#include "test_sax.hpp"
+#include "sax_event_loggers.hpp"
 #include "test_utils.hpp"
 
 using utils::SaxCountdown;