ci: add dependabot automerge workflow (#611)

Add workflow to auto-approve and auto-merge Dependabot PRs.

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Author: unional

.github/workflows/automerge-dependabot.yml

@@ -0,0 +1,43 @@
+# Inspired by https://github.com/vercel/turborepo/blob/main/.github/workflows/examples-autoapprove-and-automerge.yml
+# Auto-approves and auto-merges Dependabot PRs.
+name: Dependabot automerge
+on: pull_request
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  dependabot-approve:
+    runs-on: ubuntu-latest
+    if: |
+      always() &&
+      !contains(needs.*.result, 'failure') &&
+      !contains(needs.*.result, 'cancelled') &&
+      github.actor == 'dependabot[bot]'
+    steps:
+      - name: Dependabot metadata
+        id: metadata
+        uses: dependabot/fetch-metadata@v2
+        with:
+          github-token: '${{ secrets.GITHUB_TOKEN }}'
+      - name: Approve a PR
+        run: gh pr review --approve "$PR_URL"
+        env:
+          PR_URL: ${{github.event.pull_request.html_url}}
+          GH_TOKEN: ${{secrets.GITHUB_TOKEN}}
+  dependabot-merge:
+    needs: [dependabot-approve]
+    runs-on: ubuntu-latest
+    if: github.actor == 'dependabot[bot]'
+    steps:
+      - name: Dependabot metadata
+        id: metadata
+        uses: dependabot/fetch-metadata@v2
+        with:
+          github-token: '${{ secrets.GITHUB_TOKEN }}'
+      - name: Enable auto-merge for Dependabot PRs
+        run: gh pr merge --auto --rebase "$PR_URL"
+        env:
+          PR_URL: ${{github.event.pull_request.html_url}}
+          GH_TOKEN: ${{secrets.GITHUB_TOKEN}}