Expose Parser and add OpenAI example in usage guide

[martosaur]

Hi @benjreinhart and thank you for the library, it's amazing!

I kind of miss version 0.2.0 manual parsing, since it allowed me to use Req's callback streaming and, most importantly, make Req eventually return whole response body once the stream is complete. This is useful, for example, if any response steps expect a full body. One example is PostHog's LLMAnalytics Req plugin.

This PR just adds Parser.init/0 function, makes parse/0 and parse/1 public and adds an example in usage guide.

[benjreinhart]

Hey @martosaur, appreciate the PR!

I will happily add this ability back. However, I have some implementation preferences that differ from what is here.

1. Let's do everything through the top-level module, e.g., ServerSentEvents.parse/1 and ServerSentEvents.parse/2
2. Let's keep the parser module private with moduledoc: false
3. Can we use Parser.new instead of Parser.init? I believe new is more idiomatic here
4. Can we add mention of .parse to readme? It doesn't need to be a full big example, but maybe a simple one, similar to what was there for the older version of this library?

[martosaur]

@benjreinhart of course! Just pushed the changes

[benjreinhart]

You know, i'm so sorry, but I was wrong. It doesn't make sense to me to expose a new() function to construct a struct from another module. I think your first pass here makes sense.

Sorry to thrash, but I think moving back to exposing the parser module and using that to do the lower-level parsing is the right approach.

Thanks

[martosaur]

Haha it's all good, I left initial commit intact just in case. Please check now!

[benjreinhart]

Was this intentional? I think we can leave this here?

[martosaur]

Yes! Sorry should have highlighted this. It's identical to ServerSentEvents.event so I just made this module to use that one. It'll be something like that: