ci: opam install and run jest tests

Author: actionshrimp

.github/workflows/build.yml

@@ -72,3 +72,30 @@ jobs:
       - run: npm ci
       - run: npm run build
       - run: npm test
+
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Use Node.js ${{ matrix.node-version }}
+        uses: actions/setup-node@v3
+        with:
+          node-version: ${{ matrix.node-version }}
+          cache: 'npm'
+
+      - run: npm ci
+
+      - name: Use OCaml ${{ matrix.ocaml-compiler }}
+        uses: ocaml/setup-ocaml@v2
+        with:
+          ocaml-compiler: ${{ matrix.ocaml-compiler }}
+          opam-pin: false
+          opam-depext: false
+          cache-prefix: v2
+
+      - working-directory: js
+        run: |
+          opam pin -y -n melange-jest git+https://github.com/melange-community/melange-jest.git
+          opam pin -y -n melange-webapi git+https://github.com/melange-community/melange-webapi.git
+
+      - run: opam install ./decoders.opam ./melange-decoders.opam --with-test
+      - run: npx jest _build/default/__tests__