Split out hm/common

I went nuclear...

hm/nix/default.nix

@@ -0,0 +1,24 @@
+{ pkgs, lib, config, ... }:
+{
+  config = {
+    home.packages = with pkgs; [
+      nix-diff
+      nix-tree
+      nix-output-monitor
+    ];
+    programs.nix-index = {
+      # For non-NixOS systems
+      enable = false; # TODO Index is impossible to generate, should use https://github.com/nix-community/nix-index-database
+      # but got no luck without flakes
+      enableZshIntegration = true;
+    };
+    nix = {
+      package = lib.mkDefault pkgs.nixFlakes;
+      settings = {
+        experimental-features = [ "nix-command" "flakes" ];
+        warn-dirty = false;
+      };
+    };
+    nixpkgs.config.allowUnfree = true;
+  };
+}