Refactor vim plugins overlay

Add overlays for additions, modifications and unstable packages, and
move each additional plugin into its own additional package.

overlays/default.nix

@@ -0,0 +1,20 @@
+{ inputs, ... }:
+
+{
+  additions =
+    final: prev:
+    import ../pkgs {
+      inherit prev;
+
+      pkgs = final;
+    };
+
+  modifications = final: prev: { };
+
+  unstable-packages = final: _prev: {
+    unstable = import inputs.nixpkgs-unstable {
+      config.allowUnfree = true;
+      system = final.system;
+    };
+  };
+}