style: add treefmt Nix devshell

`haskell.nix`'s useful `devShells` function doesn't work with
`flake-parts`, which we use for universal formatting support, so let's
just create a separate Nix devshell for formatting.

Also add a handy `make format` target.

Signed-off-by: Drew Hess <src@drewhess.com>

Author: dhess

Makefile

@@ -25,10 +25,13 @@ develop-frontend:
 develop-ui-demo:
 	$(MAKE) -C primer-miso-ui develop-ui-demo
 
+format:
+	nix develop .#treefmt -c treefmt
+
 clean:
 	cabal clean
 
-.PHONY: build $(project-targets) $(package-targets) clean
+.PHONY: build $(project-targets) $(package-targets) format clean
 
 # Disabled until Weeder is fixed with haskell.nix
 

flake.nix

@@ -291,11 +291,7 @@
             };
 
           devShells = {
-            default = primerFlake.devShells.default // {
-              inputsFrom = [
-                config.treefmt.build.devShell
-              ];
-            };
+            default = primerFlake.devShells.default;
             wasm = pkgs.mkShell {
               packages = with inputs.ghc-wasm.packages.${system};
                 [
@@ -310,6 +306,11 @@
                   (pkgs.haskell-nix.tool ghcVersion "tasty-discover" { })
                 ];
             };
+            treefmt = pkgs.mkShell {
+              inputsFrom = [
+                config.treefmt.build.devShell
+              ];
+            };
           };
         };