Default `double_quotes` flag should be `chars`

https://github.com/ichiban/prolog/issues/176#issuecomment-1075505151

The current default is `codes`, which follows GNU Prolog.

https://github.com/ichiban/prolog/blob/f2029460f9a56e751e3d1e27f06a68448012460e/engine/parser.go#L628