add dev shell flake

flake.nix

@@ -0,0 +1,25 @@
+{
+  description = "Dev shell flake for servicepoint-tanks";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-24.05";
+  };
+
+  outputs =
+    { self, nixpkgs }:
+    let
+      lib = nixpkgs.lib;
+      forAllSystems = lib.genAttrs lib.systems.flakeExposed;
+    in
+    {
+      devShells = forAllSystems (system: {
+        default = import ./shell.nix {
+          inherit nixpkgs;
+          pkgs = nixpkgs.legacyPackages."${system}";
+          lib = nixpkgs.lib;
+        };
+      });
+
+      formatter = forAllSystems (system: nixpkgs.legacyPackages."${system}".nixfmt-rfc-style);
+    };
+}