Automatically merge dependabot if CI passes (#106)

Copied this from Martin repo - if dependabot creates a PR, it
automatically gets approved and merged if all CI passes.

.github/workflows/dependabot.yml

@@ -0,0 +1,27 @@
+name: Dependabot auto-merge
+on: pull_request
+
+permissions: write-all
+
+jobs:
+  dependabot:
+    runs-on: ubuntu-latest
+    if: github.actor == 'dependabot[bot]'
+    steps:
+      - name: Dependabot metadata
+        id: metadata
+        uses: dependabot/[email protected]
+        with:
+          github-token: "${{secrets.GITHUB_TOKEN}}"
+      - name: Approve Dependabot PRs
+        if: steps.metadata.outputs.update-type == 'version-update:semver-patch'
+        run: gh pr review --approve "$PR_URL"
+        env:
+          PR_URL: ${{github.event.pull_request.html_url}}
+          GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}
+      - name: Enable auto-merge for Dependabot PRs
+        if: steps.metadata.outputs.update-type == 'version-update:semver-patch'
+        run: gh pr merge --auto --squash "$PR_URL"
+        env:
+          PR_URL: ${{github.event.pull_request.html_url}}
+          GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}