build(nix): add formatting checks

Signed-off-by: seth <getchoo@tuta.io>

nix/checks.nix

@@ -0,0 +1,42 @@
+{
+  runCommand,
+  deadnix,
+  llvmPackages_18,
+  markdownlint-cli,
+  nixfmt-rfc-style,
+  statix,
+  self,
+}:
+{
+  formatting =
+    runCommand "check-formatting"
+      {
+        nativeBuildInputs = [
+          deadnix
+          llvmPackages_18.clang-tools
+          markdownlint-cli
+          nixfmt-rfc-style
+          statix
+        ];
+      }
+      ''
+        cd ${self}
+
+        echo "Running clang-format...."
+        clang-format -i --style='file' --Werror */**.{c,cc,cpp,h,hh,hpp}
+
+        echo "Running deadnix..."
+        deadnix --fail
+
+        echo "Running markdownlint..."
+        markdownlint --dot .
+
+        echo "Running nixfmt..."
+        nixfmt --check .
+
+        echo "Running statix"
+        statix check .
+
+        touch $out
+      '';
+}