Specification of the WebSocket `esm-hmr` protocol

[TomokiMiyauci]

Apart from the HMR API, the WebSocket esm-hmr protocol should also be clearly specified.

As an example, GraphQL over WebSocket protocol specification is helpful.

According to this, and based on the current implementation, it is as follows:

Message Types

hotAccept

Direction: Client -> Server

interface HotAcceptMessage {
  type: "hotAccept";
  id: string;
}

reload

Direction: Server -> Client

interface ReloadMessage {
  type: "reload";
}

update

Direction: Server -> Client

interface UpdateMessage {
  type: "update";
  url: string;
}

You may want to unify the id and url fields.