Adding direnv and nix support

With nix, you can now pull the dotnet sdk into scope with `nix-shell`, or if
you have `direnv` installed, by running `direnv allow` - it will detect the
.envrc and prompt you to do so.

If you're not using either of these, this won't affect you at all. ^^

.envrc

@@ -0,0 +1,4 @@
+# if nix is installed and dotenv allowed in dir, use it to pull dependencies
+if has nix; then
+  use nix
+fi

shell.nix

@@ -0,0 +1,5 @@
+{ pkgs ? import <nixpkgs> {} }:
+pkgs.mkShell {
+  name = "afrapay";
+  packages = with pkgs; [ dotnet-sdk_7 ];
+}