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/nvim/lua/options.lua

@@ -40,6 +40,12 @@ opt.linebreak = true
 opt.wrap = true
 
 -- Colors
+require("tokyonight").setup({
+  transparent = true,
+  styles = {
+    floats = "transparent"
+  }
+})
 vim.cmd("colorscheme tokyonight")
 vim.cmd("hi Comment guifg=#e69dc5")
 vim.cmd("hi Linenr guifg=#a672f3")