feat(themeing): Major refactor themeing for whole computer

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.

config/swaylock/config

@@ -29,6 +29,8 @@ line-color=00000000
 separator-color=00000000
 key-hl-color=c6a0f6
 
+bs-hl-color=ed8796
+
 ## Inside Colors ##
 
 inside-color=00000088