Add Dependabot for GitHub Actions and grouped security updates

.github/dependabot.yml

@@ -0,0 +1,24 @@
+version: 2
+updates:
+  # Keeps the SHA-pinned actions and their version comments current,
+  # in one grouped PR per week.
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
+    groups:
+      github-actions:
+        patterns:
+          - "*"
+  # Security updates only. Version update PRs are off (limit 0), and all
+  # security fixes for this repository land in one grouped PR.
+  - package-ecosystem: npm
+    directory: /
+    schedule:
+      interval: weekly
+    open-pull-requests-limit: 0
+    groups:
+      security:
+        applies-to: security-updates
+        patterns:
+          - "*"