fix exercise 2 and add exercise 3 in chapter 8

[kimamula]

Thanks for the great book.

I fixed 2 problems in the answers of the exercises of chapter 8.

1. Fix onmessage compilation error

The current onmessage implementation in the answer of the exercise 2 does not compile.

I implemented handle method in which switch is used for type refinement and created Data type which takes advantage of distributive conditional type to declare more appropriate type than {command: C; args: MatrixProtocol[C]['in']} for data.

2. Add an answer for the exercise 3

Added code similar to the worker thread example.
By the way, though it is described that child_process is the way to do multithreading in Node.js, it is actually for multi processing.
Node.js provides worker_threads module for multithreading.

Other thing to note

This is not a problem in exercises, but the SafeEmitter implementation in chapter 8 does not compile.

What about using interface instead of class?

interface SafeEmitter<
  Events extends Record<PropertyKey, unknown[]>
> {
  emit<K extends keyof Events>(
    channel: K,
    ...data: Events[K]
  ): boolean
  on<K extends keyof Events>(
    channel: K,
    listener: (...args: Events[K]) => void
  ): this
  on(
    channel: never,
    listener: (...data: unknown[]) => void
  ): this
}
let commandEmitter: SafeEmitter<Commands> = new EventEmitter()

Not only does this compile, but also this is more lightweight as this is a type-level wrapper and does not emit any JavaScript code.

[kimamula]

TS playground link