add fourmolu check on CI

.github/workflows/ci.yml

@@ -67,3 +67,12 @@ jobs:
       - name: Test on oldest dependencies
         if: matrix.cabal == '3.10.1.0'
         run: cabal test --prefer-oldest
+
+  format:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Check code formatting
+        uses: fourmolu/fourmolu-action@v6