Add flake.nix and .envrc for development on NixOS

Author: JonnyHaystack

.envrc.example

@@ -0,0 +1 @@
+use flake

flake.lock

@@ -0,0 +1,18 @@
+{
+  description = "HayBox firmware";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.11";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils, ... }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let pkgs = import nixpkgs { inherit system; };
+      in {
+        devShells.default = with pkgs;
+          mkShell {
+            buildInputs = [ python3 platformio python312Packages.grpcio-tools ];
+          };
+      });
+}