chore: add automerge workflow for dependabot

Author: sweatybridge

.github/workflows/automerge.yml

@@ -0,0 +1,38 @@
+# Adapted from https://blog.somewhatabstract.com/2021/10/11/setting-up-dependabot-with-github-actions-to-approve-and-merge/
+name: Dependabot auto-merge
+
+on: pull_request
+
+permissions:
+  pull-requests: write
+  contents: write
+
+jobs:
+  dependabot:
+    runs-on: ubuntu-latest
+    # Checking the actor will prevent your Action run failing on non-Dependabot
+    # PRs but also ensures that it only does work for Dependabot PRs.
+    if: ${{ github.actor == 'dependabot[bot]' }}
+    steps:
+      # This first step will fail if there's no metadata and so the approval
+      # will not occur.
+      - name: Dependabot metadata
+        id: dependabot-metadata
+        uses: dependabot/fetch-metadata@v1
+        with:
+          github-token: "${{ secrets.GITHUB_TOKEN }}"
+
+      # Here the PR gets approved.
+      - name: Approve a PR
+        if: ${{ steps.dependabot-metadata.outputs.update-type != 'version-update:semver-major' }}
+        run: gh pr review --approve "${{ github.event.pull_request.html_url }}"
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+      # Finally, this sets the PR to allow auto-merging for patch and minor
+      # updates if all checks pass
+      - name: Enable auto-merge for Dependabot PRs
+        if: ${{ steps.dependabot-metadata.outputs.update-type != 'version-update:semver-major' }}
+        run: gh pr merge --auto --squash "${{ github.event.pull_request.html_url }}"
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}