[C++] Add ParseJson(), Parser(Parser&&), update fuzzers (#6284)

- add a new method ParseJson to minimize failures during fuzzing
- add default (conditional) move-constructor for Parser
- add a new monster_fuzzer
- switch fuzzers to C++17 and `test/cpp17` generated code

include/flatbuffers/flatbuffers.h

@@ -1240,7 +1240,7 @@ class FlatBufferBuilder {
   }
 
   /// @brief Get the serialized buffer (after you call `Finish()`) as a span.
-  /// @return Returns a constructed flatbuffers::span that is a view over the 
+  /// @return Returns a constructed flatbuffers::span that is a view over the
   /// FlatBuffer data inside the buffer.
   flatbuffers::span<uint8_t> GetBufferSpan() const {
     Finished();