Add automerge configuration for dependabot

Since we have a very good CI here (100% coverage, 100% mutation
coverage), we can pretty safely allow upgrades if they pass CI.

.github/workflows/automerge-dependabot.yml

@@ -0,0 +1,17 @@
+name: Enable automerge on dependabot PRs
+
+on:
+  # See note below about using pull_request_target
+  pull_request_target:
+
+jobs:
+  automerge:
+    name: 🚀  Enable automerge on dependabot PRs
+    runs-on: ubuntu-latest
+    steps:
+      - name: 🚀  Enable automerge on dependabot PRs
+        uses: daneden/enable-automerge-action@v1
+        with:
+          # A personal access token that you have generated and saved in the
+          # repo or org’s encrypted secrets
+          github-token: ${{ secrets.DEPENDABOT_AUTO_MERGE_PERSONAL_ACCESS_TOKEN }}