refactor: update eventsource dependency and clean up code

[rossiam]

What was done

• updated eventsource to latest version
• simplified code some by pushing more directly into the main command class
• moved handleSignals function to general util module since it's not sse-specific even though that's all we're using it for now

Notes

• With the older version of eventsource, we didn't have code to make the actual requests for the data like we do now in the fetch function passed to new EventSource. Rather, we just specified extra headers and presumably, it was just using fetch under the hood. I wanted to use axios here since that's what we use everywhere else but I was unable to get axios to work so for now, I'm sticking with fetch (from undici) as show in the eventsource README.
• I expect it will be likely I will want to refactor this more before releasing the yargs branch. I'm trying to work out how to do a release now that we can't use pkg because we're using ES modules. I've found I can use Deno to build a binary which works for other commands but not this one for some reason.

References

• https://github.com/EventSource/eventsource#readme

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