Add nix support (space-wizards#115)

(Don't want to be the nix shill but I don't want to have cargo installed
on my computer.)

### Add nix support to this repo,

Can quickly set up locally with:
`nix-shell --command 'mdbook serve'`
or
`nix develop -c mdbook serve`

flake.nix

@@ -0,0 +1,16 @@
+{
+  description = "Development environment for Space Station 14";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/release-23.11";
+
+  outputs = { self, nixpkgs }:
+    let
+      forAllSystems = function:
+        nixpkgs.lib.genAttrs [ "x86_64-linux" "aarch64-linux" ]
+        (system: function nixpkgs.legacyPackages.${system});
+    in {
+      devShells = forAllSystems
+        (pkgs: { default = import ./shell.nix { inherit pkgs; }; });
+    };
+
+}

shell.nix

@@ -0,0 +1,16 @@
+{ pkgs ? import (builtins.fetchTarball {
+  url =
+    "https://github.com/nixos/nixpkgs/archive/66559cae054c9abe9f6f997c3c9720fbad8d5443.tar.gz";
+  sha256 = "sha256-2u/2oCSFH4QCVKeL/GHnse/S+rZ7aOMQeFJOIwCdBYU=";
+}) { } }:
+
+with pkgs;
+mkShell {
+  buildInputs = [
+    mdbook
+    mdbook-mermaid
+    mdbook-linkcheck
+    mdbook-admonish
+    mdbook-emojicodes
+  ];
+}