Native support for EventSource

[SamyPesse]

EventSource is a good and simple API to easily implement streaming. At the moment, it's easy to generate an event source stream from a Cloudflare Worker, but it's not easy to consume one from a Clouflare Worker.

Most polyfills are incompatible with Cloudflare Workers:

• event-source-polyfill uses fetch options that are not supported by Cloudflare, I'll publish a fork
• eventsource relies on http/https module and doesn't work well with the browserify polyfills

While it's definitely possible to make a polyfill of EventSource works on Worker, it'd be great if this API could be natively supported, especially as the API surface is light and not browser-specific.

It looks like @cloudflare/ai also consumes an EventSource API, but it's unclear what polyfill is being used there and if the Cloudflare team could publish it as an open source package.