feat(tools): add stale config

this adds a [stale](https://probot.github.io/apps/stale/) config to automatically ping on issues
that are open for more than 60 days and close them 30 days after that. this should help us clean up
old issues automatically.

.github/stale.yml

@@ -0,0 +1,13 @@
+daysUntilStale: 60
+daysUntilClose: 30
+# Issues with these labels will never be considered stale.
+exemptLabels:
+  - pinned
+  - security
+# Label to use when marking an issue as stale.
+staleLabel: stale
+# Comment to post when marking an issue as stale.
+markComment: >
+  This issue has been automatically marked as stale because it has not had
+  recent activity. It will be closed if no further activity occurs. Thank you
+  for your contributions.