pkgs: add leodagan

Leodagan is a tool written by Cyril Duval to check that a message respects the EPITA's netiquette. Having it available to the students via the packages of NixPIE would be of great help to ensure that they can check that their news and/or tickets will be netiquette compliant.

Because the main repository is just a basic python script (there was no build system to package the tool) and is not actively being maintained, I forked the main repo and added poetry as a build system as well as enhancing several aspects of the tool.