Add stale bot (#545)

Author: Nyholm

.github/workflows/stale.yml

@@ -0,0 +1,13 @@
+daysUntilStale: 180
+daysUntilClose: 28
+exemptLabels:
+  - keep-open
+# Label to use when marking an issue as stale
+staleLabel: stale
+# Comment to post when marking an issue as stale. Set to `false` to disable
+markComment: >
+  This issue has been automatically marked as stale because it has not had
+  recent activity. It will be closed after 4 weeks if no further activity occurs. Thank you
+  for your contributions.
+# Comment to post when closing a stale issue. Set to `false` to disable
+closeComment: false