feat: add issue stale workflow (#62)

.github/workflows/stale.yml

@@ -0,0 +1,14 @@
+name: 'Close stale issues'
+on:
+  schedule:
+    - cron: '30 3 * * *'
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/stale@v9
+        with:
+          stale-issue-message: 'This issue is stale because it has been open 15 days with no activity. Remove stale label or comment or this will be closed in 5 days.'
+          days-before-stale: 15
+          days-before-close: 5