Add test build (#123)

This does a test on all pull requests, to make sure that the site actually builds.

.github/workflows/tests.yml

@@ -0,0 +1,20 @@
+name: tests
+
+on: pull_request
+
+jobs:
+  test_build:
+    name: Test build
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: 'npm'
+      - name: Install dependencies
+        run: npm ci
+      - name: Build with Eleventy
+        run: npm run-script build