refactor: combine nix-settings and nixpkgs modules into one nix module

modules/nixos/features/nix.nix

@@ -0,0 +1,31 @@
+{
+  lib,
+  ...
+}:
+{
+  # allow packages with non-free licenses
+  nixpkgs.config.allowUnfree = true;
+
+  nix = {
+    gc = {
+      automatic = true;
+      dates = "weekly";
+      options = "--delete-older-than 20d";
+      persistent = true;
+    };
+    optimise = {
+      automatic = true;
+      persistent = true;
+    };
+    settings = {
+      experimental-features = [
+        "nix-command"
+        "flakes"
+      ];
+      trusted-users = [
+        "will"
+        "srv"
+      ];
+    };
+  };
+}