Polylux breking changes

[alberto-lazari]

The typst-slides package was rebranded as polylux to be elegible as an official Typst package. It has also introduced a number of breaking changes, that don't allow the compilation of the template.

Solutions

• Use a submodule to link the correct older version used for this template
• Update the code to use the updated version. This has also the benefit of using Typst's new package manager, that automatically pulls packages and keeps them at the correct version