chore: add automerge workflow (#57)

.github/workflows/automerge.yml

@@ -0,0 +1,19 @@
+name: Dependabot auto-merge
+
+on: pull_request
+
+permissions:
+  contents: write
+
+jobs:
+  dependabot:
+    runs-on: ubuntu-latest
+    if: ${{ github.actor == 'dependabot[bot]' }}
+    steps:
+      - name: Enable auto-merge for Dependabot PRs
+        run: gh pr merge --auto --squash "$PR_URL"
+        env:
+          PR_URL: ${{github.event.pull_request.html_url}}
+          # GitHub provides this variable in the CI env. You don't
+          # need to add anything to the secrets vault.
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}