chore(nix): add shell.nix file

shell.nix

@@ -0,0 +1,17 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell {
+  buildInputs = with pkgs; [
+    cargo
+    clippy
+    rustfmt
+    gtk3
+    gtk-layer-shell
+    gcc
+    openssl
+  ];
+
+  nativeBuildInputs = with pkgs; [
+    pkg-config
+  ];
+}