Add example scripts showing pub/sub usage.

[simonprickett]

Add two new scripts to the examples folder to demonstrate pub/sub usage. Call them pubsub-publisher.js and pubsub-subscriber.js.

Have the publisher publish a large number of message on a variety of channel names, and the subscriber use the PSUBSCRIBE command to subscribe to them.

Use the existing Redis Streams example scripts stream-producer.js and stream-consumer.js as guidelines, and also pay attention to the guidelines for adding an example to ensure that your contribution can be merged.

This is part of our participation in Hacktoberfest 2022. Learn how to complete this issue here: https://redis.io/community/hacktoberfest/

Need help or want to talk to us? Join us on Discord where we have a dedicated Hacktoberfest channel: https://discord.gg/ueQwKUh5Z3

If you're interested in taking this issue on, please mention @simonprickett and @SuzeShardlow in the comments and ask to be assigned.

[con-mark]

Hi @simonprickett if you assign this I can start looking at it

[simonprickett]

@con-mark I've assigned this to you, looking forward to seeing your PR in the next couple of days. Have fun and please check out our guidelines:

https://redis.io/community/hacktoberfest/