ci: run dprint fmt

.github/workflows/fmt.yml

@@ -0,0 +1,33 @@
+# Check formatting of toml/md files using dprint.
+# Rust format checking is done in the CI workflow.
+
+name: fmt
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  fmt:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Cache dprint
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.dprint
+            ~/.cache/dprint
+          key: dprint
+
+      - name: Install dprint
+        run: |
+          if [ ! -f $HOME/.dprint/bin/dprint ]; then
+            curl -fsSL https://dprint.dev/install.sh | sh
+          fi
+          echo $HOME/.dprint/bin >> $GITHUB_PATH
+
+      - run: dprint check