Run tests in CI

.github/workflows/test.yaml

@@ -0,0 +1,19 @@
+name: Test
+
+on:
+  pull_request:
+
+jobs:
+  test:
+    name: Run type checker and tests
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+      - name: Use Node.js 20.x
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20.x
+      - run: npm ci
+      - run: npm typecheck
+      - run: npm test