build(nix): add dev environment flake

Basic flake to enable development under nixos. Not really flushed out,
but in a good enough state to add it. Probably needs improvement in the
future.

Refs: #409

Author: Xenira

.envrc

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

.gitignore

@@ -3,4 +3,5 @@ Cargo.lock
 /.vscode
 /.idea
 /tmp
-expand.rs
+/.direnv
+expand.rs

flake.lock

@@ -0,0 +1,40 @@
+{
+  description = "ext-php-rs dev environment";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+    rust-overlay = {
+      url = "github:oxalica/rust-overlay";
+      inputs = {
+        nixpkgs.follows = "nixpkgs";
+      };
+    };
+  };
+
+  outputs =
+    { nixpkgs, rust-overlay, ... }:
+    let
+      system = "x86_64-linux";
+      overlays = [ (import rust-overlay) ];
+      pkgs = import nixpkgs { inherit system overlays; };
+      php-dev = pkgs.php.unwrapped.dev;
+    in
+    {
+      devShells.${system} = {
+        default = pkgs.mkShell {
+          buildInputs = with pkgs; [
+            php
+            php-dev
+            libclang.lib
+            clang
+          ];
+
+          nativeBuildInputs = [ pkgs.rust-bin.stable.latest.default ];
+
+          shellHook = ''
+            export LIBCLANG_PATH="''$LIBCLANG_PATH ${pkgs.libclang.lib}/lib"
+          '';
+        };
+      };
+    };
+}