feat: add default shell

We're starting to need a few packages which people are less likely to
have on their system. Let's provide an environment where everyone can
quickly install everything we need.

shells/default/default.nix

@@ -0,0 +1,11 @@
+{
+  mkShell,
+  reuse,
+  deploy-rs,
+}:
+mkShell {
+  packages = [
+    reuse # Used to provide licenses & copyright attribution
+    deploy-rs # Used to deploy to our servers
+  ];
+}