Adds catppuccin nix flake for themeing lots of programs easily. This
enables proper themeing for:
* zathura
* imv
* fcitx (declarative settings)
* lazygit
* btop
* gtk
It also pulls in the home-manager sessionVars which is necessary to
theme applications.
The session vars change also reduces the number of env-vars that need to
be set in the zshrc due to them being set declaratively elsewhere.
