ci: update dependabot config to be quarterly, simplify auto-merge

Author: domoritz

.github/dependabot.yml

@@ -3,8 +3,8 @@ updates:
 - package-ecosystem: npm
   directory: "/"
   schedule:
-    interval: monthly
+    interval: quarterly
 - package-ecosystem: github-actions
   directory: "/"
   schedule:
-    interval: monthly
+    interval: quarterly

.github/workflows/merge-dependabot.yml

@@ -0,0 +1,23 @@
+name: Dependabot auto-merge
+on: pull_request
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  dependabot:
+    runs-on: ubuntu-latest
+    if: github.event.pull_request.user.login == 'dependabot[bot]' && github.repository == 'vega/react-vega'
+    steps:
+      - name: Dependabot metadata
+        id: metadata
+        uses: dependabot/fetch-metadata@v2
+        with:
+          github-token: '${{ secrets.GITHUB_TOKEN }}'
+      - name: Enable auto-merge for Dependabot PRs
+        if: steps.metadata.outputs.update-type == 'version-update:semver-patch' || steps.metadata.outputs.update-type == 'version-update:semver-minor'
+        run: gh pr merge --auto --squash "$PR_URL"
+        env:
+          PR_URL: ${{github.event.pull_request.html_url}}
+          GH_TOKEN: ${{secrets.GITHUB_TOKEN}}