Add ocamlformat to CI

.github/workflows/ci.yml

@@ -1,7 +1,27 @@
 name: ci
 on: [push]
 jobs:
+  ocamlformat:
+    runs-on: ubuntu-22.04
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Setup OCaml
+        uses: ocaml/setup-ocaml@v2
+        with:
+          ocaml-compiler: 4.14.x
+      - name: Install dependencies
+        run: |
+          sudo apt-get update
+          opam depext --install --yes dune
+          opam install --yes ocamlformat=0.26.2
+      - name: Run format check
+        run: |
+          # needs `opam exec --` as `dune` is not in the shell PATH
+          opam exec -- dune fmt --preview
+
   install-ubuntu:
+    needs: ocamlformat
     runs-on: ubuntu-22.04
     strategy:
       fail-fast: false
@@ -45,6 +65,7 @@ jobs:
             _build/default/_doc/_html/kappa-library
 
   python:
+    needs: ocamlformat
     runs-on: ubuntu-22.04
     steps:
       - name: Checkout
@@ -73,6 +94,7 @@ jobs:
         run: opam exec -- nosetests -v tests/kappy
 
   js:
+    needs: ocamlformat
     runs-on: ubuntu-22.04
     steps:
       - name: Checkout
@@ -108,6 +130,7 @@ jobs:
             Kappapp.tar.gz
 
   macos:
+    needs: ocamlformat
     runs-on: macos-13
     steps:
       - name: Checkout
@@ -139,6 +162,7 @@ jobs:
             Kappapp.app.zip
 
   windows:
+    needs: ocamlformat
     runs-on: windows-2022
     steps:
       - name: Checkout