Add stale-bot action (cross-language-cpp#144)

With it, we have a democratic robot that threads all issues and PRs equally, by definition, automatically.

After 60 days of inactivity, a message is triggered
After 7 additional days, the issue or PR gets closed,

Closed items can be reopened if ever needed.
Auto-closed issues and PRs get the label stale so we can review staled items if needed.
Exceptions: If a ticket / PR has the label idea, it will not be handled by this bot.

This PR activates the stale bot for the generator repository.

.github/stale.yml

@@ -0,0 +1,17 @@
+# Number of days of inactivity before an issue becomes stale
+daysUntilStale: 60
+# Number of days of inactivity before a stale issue is closed
+daysUntilClose: 7
+# Issues with these labels will never be considered stale
+exemptLabels:
+  - idea
+
+# 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 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