Fix interpretation of 'nan(number)' by the idl_parser (#5810)

* Parser reject "nan(n)" string as it does with nan(n)

* Adjust scalar fuzzer to ignore '$schema' substrings

- Scalar fuzzer ignores '$schema' substrings at the input
- Added 'scalar_debug' target to simplify research of fuzzed cases

* Improve formatting of './tests/fuzzer/CMakeLists.txt'

tests/fuzzer/flatbuffers_parser_fuzzer.cc

@@ -26,7 +26,8 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
   if (size < 3) return 0;
   const uint8_t flags = data[0];
   // normalize to ascii alphabet
-  const int extra_rep_number = data[1] >= '0' ? (data[1] - '0') : 0;
+  const int extra_rep_number =
+      std::max(5, (data[1] < '0' ? (data[1] - '0') : 0));
   data += 2;
   size -= 2;  // bypass