Add nix app for Birdie

Signed-off-by: Naomi Roberts <[e-mail]>

README.md

···
 ## Development
 
 ```sh
-gleam run   # Run the project
-gleam test  # Run the tests
+gleam run -m birdie   # Run birdie
+gleam test            # Run the tests
+```
+
+### Nix
+```sh
+nix run .#birdie   # Run birdie
+nix run .#test     # Run the tests
 ```

flake.nix

···
           '';
         })}/bin/test";
       };
+      birdie = {
+        type = "app";
+        program = "${(pkgs.writeShellApplication {
+          inherit runtimeInputs;
+          name = "birdie";
+          text = ''
+            ${pkgs.gleam}/bin/gleam run -m birdie
+          '';
+        })}/bin/birdie";
+      };
     });
   };
 }