build: add GitHub Action to update tools modules

Update ESLint, Babel, remark, and so on. Run once a week.

.github/workflows/tools.yml

@@ -0,0 +1,29 @@
+name: "tools update"
+on:
+  schedule:
+    # Run once a week at 00:05 AM UTC on Saturday.
+    - cron: '5 0 * * 6'
+
+  workflow_dispatch:
+
+jobs:
+  tools_update:
+    if: github.repository == 'nodejs/node'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - run: tools/update-eslint.sh
+      - run: tools/update-babel-eslint.sh
+      - run: |
+          (cd tools/lint-md && rm -rf package-lock.json node_modules && npm install --ignore-scripts)
+          make lint-md-rollup
+      - uses: gr2m/create-or-update-pull-request-action@v1  # Create a PR or update the Action's existing PR
+        env:
+          GITHUB_TOKEN: ${{ secrets.GH_USER_TOKEN }}
+        with:
+          author: Node.js GitHub Bot <github-bot@iojs.org>
+          body: "This is an automated update of dependencies in the tools directory."
+          branch: "actions/tools-update"  # Custom branch *just* for this Action.
+          commit-message: "tools: update tools packages"
+          labels: tools
+          title: "tools: update tools packages"