add prettier workflow too

.github/workflows/prettier.yaml

@@ -0,0 +1,29 @@
+# yaml-language-server: $schema=https://json.schemastore.org/github-workflow.json
+name: Prettier checks
+on:
+  push:
+    branches: ['main', '67-add-svelte-check-to-github-workflows---styleguide']
+  pull_request:
+    branches: ['main', '67-add-svelte-check-to-github-workflows---styleguide']
+
+jobs:
+  prettier:
+    name: Prettier
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Use Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: 'npm'
+
+      - name: Install Dependencies
+        run: npm ci
+
+      - name: Check formatting with Prettier
+        run: npm run prettier:check:ci