feat: migrate zellij from omakub

[Arash-Afshar]

I made two main changes in the migration:

1. The theme names are all "main" so that when I change the symlink it works without having to change the zellij config as well.
2. Deleted the different options in themes/catppuccin/zellij.kdl and kept only the first one

[dhh]

I actually didn't include zellij in Omarchy on purpose. I've found that with a tiling window manager, it just isn't as necessary. I end up just hitting Super+Return whenever I need a new terminal tile. Besides, we'll be switching to Ghostty in a few months, which has its own built-in panes and tabs for folks who still want that.