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'

include/flatbuffers/flatbuffers.h

@@ -1795,8 +1795,7 @@ class FlatBufferBuilder {
       return a.KeyCompareLessThan(&b);
     }
 
-   private:
-    StructKeyComparator &operator=(const StructKeyComparator &);
+    FLATBUFFERS_DELETE_FUNC(StructKeyComparator &operator=(const StructKeyComparator &))
   };
   /// @endcond
 

include/flatbuffers/idl.h

@@ -891,6 +891,7 @@ class Parser : public ParserState {
   FLATBUFFERS_CHECKED_ERROR TokenError();
   FLATBUFFERS_CHECKED_ERROR ParseSingleValue(const std::string *name, Value &e,
                                              bool check_now);
+  FLATBUFFERS_CHECKED_ERROR ParseFunction(const std::string *name, Value &e);
   FLATBUFFERS_CHECKED_ERROR ParseEnumFromString(const Type &type,
                                                 std::string *result);
   StructDef *LookupCreateStruct(const std::string &name,