chore: add action for format checking

.forgejo/workflows/format_pull.yaml

@@ -0,0 +1,18 @@
+name: format
+
+on:
+  pull_request:
+
+jobs:
+  test:
+    runs-on: nixos-latest
+    steps:
+      - name: checkout
+        uses: actions/checkout@v4
+        with:
+          ref: 'main'
+      - name: dev
+        uses: actions/nix/develop@main
+        with:
+          flake: .
+          run: treefmt