CI: Add dependabot config for GitHub Actions

This should help keep GitHub Actions versions up-to-date.

.github/dependabot.yml

@@ -0,0 +1,18 @@
+version: 2
+
+updates:
+  # Update GitHub Actions versions in workflows
+  - package-ecosystem: "github-actions"
+    # Workflow files stored in the
+    # default location of `.github/workflows`
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    allow:
+      - dependency-type: all
+    open-pull-requests-limit: 10
+    pull-request-branch-name:
+      separator: "-"
+    commit-message:
+      prefix: "CI: "
+      include: "scope"