[dev-server] add ESM HMR plugin

[LarsDenBakker]

We should add a plugin for implementing ESM based HMR, based on the specs defined here: https://github.com/pikapkg/esm-hmr

HMR needs two way communication. We could implement with the current system using EventSource and fetch, but it is probably simpler to do #500 first

[LarsDenBakker]

We now have a web socket API https://modern-web.dev/docs/dev-server/writing-plugins/web-sockets/ so this is ready to be picked up.

[LarsDenBakker]

WIP Implementation at #685

[LarsDenBakker]

It's now released: https://modern-web.dev/docs/dev-server/plugins/hmr/