Add Stale bot configuration (#1575)

.github/stale.yml

+exemptProjects: true
+exemptMilestones: true
+
+closeComment: >
+  Closing this issue automatically because it has not had any activity since
+  it has been marked as stale. If you think it is still relevant and should
+  be addressed, feel free to open a new one.
+
+pulls:
+  markComment: >
+    This pull request 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.
+
+  closeComment: >
+    Closing this pull request automatically because it has not had any activity since
+    it has been marked as stale. If you think it is still relevant and should
+    be addressed, feel free to open a new one.