bug#47119: 28.0.50; [patch][Dired] new user option for compressing dir suffix

[Eli Zaretskii]

> Date: Sat, 13 Mar 2021 11:55:48 +0800
> From:  Lin Sun via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> A new user option `dired-compress-files-default-suffix' for
> controlling the default format for compressing directory.

Thanks, but wouldn't it be better to use symbols instead of strings?
That way, we could prevent typos easier, and also offer completion for
setting the option interactively.

Also, doesn't the user manual need to be changed as well?