fetch-event-source VS better-sse

* `text/event-stream` relies on a modified version of [Azure's fetch-event-source](https://github.com/Azure/fetch-event-source). They handle errors and chunking better than I was. New version has been tested out to 4 billion fragments rendered on a single page with no errors.

The EventSource Web API for handling SSEs (built into most modern browsers) unfortunately supports only GET requests, which was quite limiting when a POST request with larger body JSON data was required. As an alternative, you can use the Fetch API or a ready library like Microsoft’s Fetch Event Source.

Aforementioned limitations are taken care with a specialised SSE client (inspired by the awesome @microsoft/fetch-event-source) and two separate connection modes: the HTTP/1 safe "single connection mode" that uses a single SSE connection for receiving events with separate HTTP requests dictating the behaviour, and the HTTP/2+ "distinct connections mode" that uses distinct SSE connections for each GraphQL operation, accommodating the parameters in the request itself.

I wrote a library for using SSE with Node and have read the spec front-to-back dozens of times when I was working on it, so I have a fairly good understanding if you have any more unanswered questions!

It actually originally started out as an Express middleware, but I then rewrote it to use the underlying Node HTTP native module so it could work with most/all frameworks, and then relied on creating a Recipes section in the docs to demonstrate how to create your own abstractions.

I've been working on a library that makes working with server-sent events easier on the server-side that can hopefully be relevant here.