Add bot to close stale issues (#864)

.github/workflows/stale-bot.yml

@@ -0,0 +1,25 @@
+name: Close stale issues
+on:
+  schedule:
+    - cron: "30 1 * * *"
+
+jobs:
+  close-stale:
+    runs-on: ubuntu-latest
+    permissions:
+      issues: write
+      pull-requests: write
+    steps:
+      - uses: actions/stale@v5
+        with:
+          days-before-issue-stale: 90
+          days-before-issue-close: 30
+          stale-issue-label: "stale"
+          stale-issue-message: "This issue is stale because it has been open for 90 days with no activity."
+          close-issue-message: "This issue was closed because it has been inactive for 30 days since being marked as stale."
+          exempt-issue-labels: "pr-welcome"
+          days-before-pr-stale: 90
+          days-before-pr-close: -1
+          stale-pr-label: "stale"
+          stale-pr-message: "This PR is stale because is has been open for 90 days with no activity."
+          repo-token: ${{ secrets.GITHUB_TOKEN }}