Write a frontend client library for JavaScript in Kotlin/JS without any dependencies

[goncalossilva]

Unfortunately, the KMP library is built on top of ktor, coroutines, and a few other libraries that bloat the Bundle. See KTOR-1084.

But the functionality is limited enough that we can replicate most of it using EventSource, or even fetch (and a dash of local storage). It should be tiny. If we tackle it by adding another (independent) source set wihtout any dependencies and keep it in Kotlin/JS, it's likely we can reuse all tests, hence not losing all benefits of a consolidated implementation.

This doesn't apply to the backend client library, where size is a significantly less relevant concern.