Add minimal github action for yarn tests

.github/workflows/tests.yml

@@ -0,0 +1,31 @@
+name: test stan-playground
+
+on:
+  pull_request:
+    branches: ['main']
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: 'tests'
+  cancel-in-progress: true
+
+jobs:
+  frontend-tests:
+    name: yarn tests
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Set up Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: 'yarn'
+          cache-dependency-path: gui/yarn.lock
+      - name: Install dependencies
+        run: cd gui; yarn
+      - name: Build
+        run: cd gui; yarn test