feat(wpcarro/marcus): Prefer light to xbacklight

TL;DR:
- Enable light (let there be light!)
- Refactor screen-brightness.el

TIL: `nixos-options programs.enable.light` outputs docs from the command-line
Change-Id: Ib60a9b3c9b7f6f96fda29fce1a97ca9995822048
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4385
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>

users/wpcarro/nixos/marcus/default.nix

@@ -67,7 +67,10 @@ in {
   users.mutableUsers = true;
   users.users.wpcarro = {
     isNormalUser = true;
-    extraGroups = [ "wheel" ];
+    extraGroups = [
+      "wheel"
+      "video" # needed to control the screen brightness
+    ];
     shell = pkgs.fish;
   };
 
@@ -83,7 +86,10 @@ in {
     };
   };
 
-  programs.fish.enable = true;
+  programs = {
+    fish.enable = true;
+    light.enable = true;
+  };
 
   environment.systemPackages = with pkgs; [
     alacritty