add option to allow writing the prefix as well

include/flatbuffers/idl.h

@@ -376,6 +376,7 @@ struct IDLOptions {
   std::string go_namespace;
   bool reexport_ts_modules;
   bool protobuf_ascii_alike;
+  bool prefix_size;
 
   // Possible options for the more general generator below.
   enum Language {
@@ -428,6 +429,7 @@ struct IDLOptions {
       skip_flatbuffers_import(false),
       reexport_ts_modules(true),
       protobuf_ascii_alike(false),
+      prefix_size(false),
       lang(IDLOptions::kJava),
       mini_reflect(IDLOptions::kNone),
       lang_to_generate(0) {}