flake: Add codespell

Nix users may not have `codespell` installed globally.
Introduce it in the `devShell` so they have it locally in `heartwood`.

flake.nix

@@ -362,6 +362,7 @@
           cargo-watch
           cargo-nextest
           cargo-semver-checks
+          codespell
           ripgrep
           sqlite
         ];