add (overlays): add latest & old packages overlays

overlays/default.nix

@@ -0,0 +1,21 @@
+{
+  systemArch,
+  nixpkgs-old,
+  nixpkgs-latest,
+  packagesConfig,
+}:
+{
+  latestPackages = final: _: {
+    latest = import nixpkgs-latest {
+      system = systemArch;
+      config = packagesConfig;
+    };
+  };
+
+  oldPackages = final: _: {
+    old = import nixpkgs-old {
+      system = systemArch;
+      config = packagesConfig;
+    };
+  };
+}