Create JSEnv for miniflare

[armanbilge]

https://miniflare.dev/
https://github.com/scala-js/scala-js-js-envs/

This is a yak, but seems conceptually similar to the JSDOM environment: a sandbox inside Node.js. Once we have this we can run tests suites as if they are running inside a worker which will be phenomenal.

We'll want to publish this one so downstreams can also test their workers.