shells: add some basic devshells

modules/shells/default.nix

@@ -0,0 +1,32 @@
+{
+  inputs,
+  ...
+}:
+let
+  supportedSystems = [
+    "x86_64-linux"
+    "aarch64-linux"
+  ];
+
+  forAllSystems = f: inputs.nixpkgs.lib.genAttrs supportedSystems (system: f system);
+in
+{
+  flake.devShells = forAllSystems (
+    system:
+    let
+      pkgs = import inputs.nixpkgs { inherit system; };
+    in
+    {
+      default = pkgs.mkShell {
+        packages = with pkgs; [ atool ];
+      };
+
+      zig = pkgs.mkShell {
+        packages = [
+          inputs.zig-overlay.packages.${system}.master
+          inputs.zls.packages.${system}.zls
+        ];
+      };
+    }
+  );
+}