Support integrating with plasma-manager for theming Plasma, Konsole, ...

[pluiedev]

Theming KDE apps, including Plasma itself, has always been a tricky matter. Since KDE always expects its configuration files to be freely writable, the typical approach to simply make the file a symlink of a generated config file in the store is not viable. However, catppuccin-nix could integrate with plasma-manager to support KDE theming, as it already has all the foundations needed to correctly handle KDE config files.

However, there is one problem with supporting plasma-manager — it has quite a hefty size, and not everyone requires using it. Since flakes as of now don't support optional dependencies, that means we could potentially end up needing a different version of the main flake that does offer plasma-manager support. Would that amount of work really be worth it to support KDE?