Limit the number of unanswered Typings Installer requests (#18321)

* Document ThrottledOperations.schedule

* Limit the number of unanswered typings installer requests

If we send them all at once, we (apparently) hit a buffer limit in the
node IPC channel and both TS Server and the typings installer become
unresponsive.

* Fix lint issues

* Add explanatory comment

Author: amcasey