Web/WhatWG Streams API Support

[pojntfx]

Currently, this library supports the legacy NodeJS streams API (see https://github.com/kriszyp/cbor-x?tab=readme-ov-file#streams), but not the newer Web/WhatWG streams API (see https://developer.mozilla.org/en-US/docs/Web/API/Streams_API). While it is possible on NodeJS to convert from the legacy API to the the new web streams API with Readable.toWeb()/Writable.toWeb(), neither of these APIs are available in the browser, making it hard to use cbor-x's streaming decode/encode support correctly (with support for backpressure etc.) when reading from a Readable stream such as a fetch request body or a WebTransport stream.

Adding support for this new (and arguable simpler to implement) streams API natively would greatly simplify this library's use in the browser.

[MeirionHughes]

As an aside, if you add 'readable-stream' (node's v18 streams) to your bundler and import from there instead of node:streams, you can use the toWeb, fromWeb helpers in the browser.

[bradisbell]

I worked up a decoder stream, following the Node.js stream code as a reference. I haven't done much testing so I'm not submitting a pull request for now, but let me know if you see any issues...

import { Decoder } from 'cbor-x';

export default class CborDecoderStream {
  #decoder;
  #incompleteBuffer;

  #transform(chunk, controller) {
    if (this.#incompleteBuffer) {
      const newBuffer = this.#incompleteBuffer.length + chunk.length;
      newBuffer.set(this.#incompleteBuffer);
      newBuffer.set(chunk, this.#incompleteBuffer.byteLength);
      chunk = newBuffer;
    }

    let values;
    try {
      values = this.#decoder.decodeMultiple(chunk);
    } catch(e) {
      if (e.incomplete) {
        this.#incompleteBuffer = chunk.slice(e.lastPosition);
        values = e.values;
      } else {
        controller.error(e);
      }
    } finally {
      for (const value of values || []) {
        controller.enqueue(value);
      }
    }
  }

  constructor(opts) {
    opts = opts || {};
    opts.structures = []; // TODO: What is this for??
    this.#decoder = opts.decoder || new Decoder(opts);

    return new TransformStream({
      transform: this.#transform.bind(this)
    })
  }
}