nix: Add treefmt to devShell

This also adds the formatters themselves to the devShell.

Resolves #36

flake.nix

@@ -42,7 +42,10 @@
         };
 
         devShells.default = pkgs.mkShell {
-          inputsFrom = [ self'.devShells.rust ];
+          inputsFrom = [
+            self'.devShells.rust
+            config.treefmt.build.devShell
+          ];
           packages = [
             pkgs.cargo-watch
             config.process-compose.cargo-doc-live.outputs.package