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/fuzzer-parse_bjdata.cpp

@@ -51,12 +51,8 @@ The provided function `LLVMFuzzerTestOneInput` can be used in different fuzzer
 drivers.
 */
 
-#include <cassert>
-#include <nlohmann/json.hpp>
 #include "fuzzer_common.hpp"
 
-using nlohmann::json;
-
 // value-stable comparison for the round-trip checks below; see the note
 // above on why this compares dump()s rather than the json values directly
 static bool is_value_stable(const json& lhs, const json& rhs)