lib: adding perf notes js_stream_socket.js

Leaving var in place of let for performance optimization
in short loops in hot paths. Added comments explaining why.

PR-URL: https://github.com/nodejs/node/pull/30415


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>