Add GitHub workflow to check for stale PRs and issues (#62)

Author: lucacome

.github/workflows/stale.yml

@@ -0,0 +1,20 @@
+name: 'Close stale issues and PRs'
+on:
+  schedule:
+    - cron: '30 1 * * *'
+
+jobs:
+  stale:
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/stale@v3
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          stale-issue-message: 'This issue is stale because it has been open 60 days with no activity. Remove stale label or comment or this will be closed in 7 days.'
+          stale-pr-message: 'This PR is stale because it has been open 60 days with no activity. Remove stale label or comment or this will be closed in 7 days.'
+          close-issue-message: 'This issue was closed because it has been stalled for 7 days with no activity.'
+          close-pr-message: 'This PR was closed because it has been stalled for 7 days with no activity.'
+          stale-issue-label: 'stale'
+          stale-pr-label: 'stale'
+          exempt-all-pr-assignees: true
+          operations-per-run: 100