Consider adding 'Symbol.tokenSpecies' and 'Symbol.tokenSourceSpecies'

[rbuckton]

There is a note in EXTENSIBILITY.md indicating that we could simplify extensibility for CancellationTokenSource and CancellationToken by defining species symbols that link to the constructors for the entangled types:

class CancellationTokenSource {
  #token;

  ...

  get token() {
    if (!#token) {
      const C = this.constructor[Symbol.tokenSpecies];
      #token = new C(this);
    }
    return #token;
  }

  static get [Symbol.tokenSpecies]() { return CancellationToken; }
}

class CancellationToken {
  static #none;
  static #canceled;

  ...

  static get none() {
    if (!#none) {
      const C = this[Symbol.tokenSourceSpecies];
      const source = new C();
      source.close(); // cannot be canceled
      #none = source.token;
    }
    return #none;
  }

  static get canceled() {
    if (!#canceled) {
      const C = this[Symbol.tokenSourceSpecies];
      const source = new C();
      source.cancel(); // already canceled
      #canceled = source.token;
    }
    return #canceled;
  }

  static get [Symbol.tokenSourceSpecies]() { return CancellationTokenSource; }
}

[rbuckton]

Closing this issue as the proposal no longer specifies cancellation behavior. See #22 for the current thinking regarding cancellation.