modules/home-manager: add fastfetch

I'm trying a slightly different module defining approach that doesn't need to rewrite the path. I have not used it elsewhere so that I can take the time to ruminate on it. Once I decide, I'll make a treewide change or simply refactor this to match the others.

modules/home-manager/programs/fastfetch/module.nix

@@ -0,0 +1,19 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+let
+  path = lib.splitString "." <| "my.programs.fastfetch";
+  cfg = lib.getAttrFromPath path config;
+in {
+  options = lib.setAttrByPath path {
+    enable = lib.mkDefaultEnableOption "fastfetch";
+    package = lib.mkPackageOption pkgs "fastfetch" {};
+  };
+
+  config = lib.mkIf cfg.enable {
+    home.packages = [ cfg.package ];
+  };
+}