createPublicChannel should join channel if the name already exists

[valeriansaliou]

The JSDoc for createPublicChannel on the JS SDK says that: https://gist.github.com/nesium/f1c21dfb0d345b8c223017f418343a4e#file-prose_sdk_js-d-ts-L482

I'd expect that calling eg. createPublicChannel("support") where #support already exists, would let me join the channel, as the JSDoc states. However, I'm getting this error: A public channel with the chosen name exists already.

[nesium]

I admit that the documentation doesn't match the actual behavior. It is sometimes hard to design the API without seeing the actual UI. I've now changed my opinion a bit about the behavior and think that it could be confusing if you were thrown into an existing channel without any notice when you thought that you were creating a brand new channel instead.

From that POV I think Slack's interface makes total sense:

If we'd want to add a bit of convenience we could still add another button that would allow to join the existing channel.

What do you think? Change the documentation or the behavior of the method?

[valeriansaliou]

Sounds good! I'll also need a method to check if a channel already exists, returning eg. a boolean or JID.

[nesium]

Added Client::findPublicChannelByName

As the name suggests finds only public channels. Refs prose-im/prose-pod-system#4