Auto-approve dependabot PRs

There is very little to be gained by having to approve those PRs, and
it is a lot of UI interaction to get them approved.
They still will need to be merged manually regardless, so they might
as well be approved by a bot.

.github/workflows/dependabot.yml

@@ -0,0 +1,21 @@
+name: Dependabot auto-approve
+on: pull_request
+
+permissions:
+  pull-requests: write
+
+jobs:
+  dependabot:
+    runs-on: ubuntu-latest
+    if: ${{ github.actor == 'dependabot[bot]' }}
+    steps:
+      - name: Dependabot metadata
+        id: metadata
+        uses: dependabot/fetch-metadata@v1.1.1
+        with:
+          github-token: "${{ secrets.GITHUB_TOKEN }}"
+      - name: Approve a PR
+        run: gh pr review --approve "$PR_URL"
+        env:
+          PR_URL: ${{github.event.pull_request.html_url}}
+          GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}