manager: Add nix shell

This nix shell will also be used as the CI environment for this tool.

techtree-manager/shell.nix

@@ -0,0 +1,19 @@
+# From 2025-05-21
+let pkgs = import (fetchTarball("https://github.com/NixOS/nixpkgs/archive/36ecfe6216f0aa7f2a1ffe5aafc2c0eae6c8cdcf.tar.gz")) {};
+
+in pkgs.mkShell {
+  buildInputs = [
+    # Rust
+    pkgs.cargo
+    pkgs.rustc
+    pkgs.rustfmt
+
+    # Dependencies
+    pkgs.openssl
+  ];
+
+  shellHook = ''
+    export OPENSSL_DIR="${pkgs.openssl.dev}"
+    export OPENSSL_LIB_DIR="${pkgs.openssl.out}/lib"
+  '';
+}