Validate UTF-8 by default when parsing IDL. Support Unicode values > U+FFFF in parse

include/flatbuffers/idl.h

@@ -348,6 +348,7 @@ struct IDLOptions {
   bool escape_proto_identifiers;
   bool generate_object_based_api;
   bool union_value_namespacing;
+  bool allow_non_utf8;
 
   // Possible options for the more general generator below.
   enum Language { kJava, kCSharp, kGo, kMAX };
@@ -370,6 +371,7 @@ struct IDLOptions {
       escape_proto_identifiers(false),
       generate_object_based_api(false),
       union_value_namespacing(true),
+      allow_non_utf8(false),
       lang(IDLOptions::kJava) {}
 };