Expose enabling listescape plugin

By default, dovecot disallows some characters in the names of mailboxes/folders, such as / and ..

The Impact of the hierarchySeparator is noted in the documentation, where it also mentions folders containing the character ..

However, with the default simple-mailserver setup, creating such folders will fail.

One way to alleviate this is by enabling the listescape plugin. This can be done by manually setting

services.dovecot2.mailPlugins.globally.enable = [ "listescape" ];

However, it would be nice if this option was directly exposed by nixos-mailserver.