flake: add a Markdown formatting check; add Deno to formatting wrapper

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I05593a413696ac3c6f03b2108f7ddd816a6a6964

flake.nix

@@ -57,7 +57,7 @@
 
       perSystem = {pkgs, ...}: {
         # Provides the default formatter for 'nix fmt', which will format the
-        # entire tree with Alejandra. The wrapper script is necessary due to
+        # entire Nix source with Alejandra. The wrapper script is necessary due to
         # changes to the behaviour of Nix, which now encourages wrappers for
         # tree-wide formatting.
         formatter = pkgs.writeShellApplication {
@@ -66,11 +66,15 @@
           runtimeInputs = [
             pkgs.alejandra
             pkgs.fd
+            pkgs.deno
           ];
 
           text = ''
             # Find Nix files in the tree and format them with Alejandra
-            fd "$@" -t f -e nix -x alejandra -q '{}'
+            fd "$@" -t f -e nix -x nixfmt -q '{}'
+
+            # Same for Markdown files, but with deno
+            fd "$@" -t f -e md -x deno fmt '{}'
           '';
         };
 
@@ -81,7 +85,16 @@
           # This can be initiated with `nix build .#checks.<system>.nix-fmt`
           # or with `nix flake check`
           nix-fmt = pkgs.runCommand "nix-fmt-check" {nativeBuildInputs = [pkgs.alejandra];} ''
-            alejandra --check ${self} < /dev/null | tee $out
+            alejandra --check ${self} < /dev/null
+            touch $out
+          '';
+
+          # Check if Markdown sources are properly formatted
+          # This can be initiated with `nix build .#checks.<system>.md-fmt`
+          # or with `nix flake check`
+          md-fmt = pkgs.runCommand "md-fmt-check" {nativeBuildInputs = [pkgs.deno];} ''
+            deno fmt --check ${self} --ext md < /dev/null
+            touch $out
           '';
         };
       };