Convert devShell to nix flake

Author: john-shaffer

.envrc

@@ -1 +1,3 @@
-use nix
+#!/bin/sh
+
+use flake

default.nix

@@ -0,0 +1,17 @@
+{
+  description = "wp2static-addon-s3";
+
+  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:
+      with import nixpkgs { inherit system; };
+      with pkgs; {
+        devShells.default = mkShell {
+          buildInputs = [ php phpPackages.composer shellcheck zip ];
+        };
+      });
+}