Add GitHub Action to check formatting.

.github/workflows/format-check.yml

@@ -0,0 +1,18 @@
+name: Check formatting
+
+on: push
+
+jobs:
+  check-formatting:
+    name: Check formatting
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-node@v3
+        with:
+          node-version: 18.x
+          cache: npm
+      - run: |
+          npm install
+      - run: |
+          npm run format-check