Refactor codebase to use nix modules

programs/fish/functions/fish_prompt.fish

@@ -1,13 +0,0 @@
-set -l last_command_status $status
-
-set -l symbol 'τ'
-
-set -l normal_color (set_color normal)
-set -l symbol_color (set_color blue -o)
-set -l error_color (set_color red -o)
-
-if test $last_command_status -eq 0
-    echo -n -s $symbol_color $symbol " " $normal_color
-else
-    echo -n -s $error_color $symbol " " $normal_color
-end