[whatwg/streams] Use `async_sequence` in `ReadableStream.from` (PR #1372)

Replace our manual async iterator logic with the new Web IDL `async_sequence<T>` type and its associated operations (added in https://github.com/whatwg/webidl/pull/1397).

This comes with one behavioral change: `ReadableStream.from("abc")` used to iterate over the Unicode code points of the string (per [`String.prototype[Symbol.iterator]`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/Symbol.iterator)), but this will now throw a `TypeError` instead. This is covered by the new WPTs.

Developers who want to create a `ReadableStream` from a string should update their code to either:
* `ReadableStream.from(["abc"])` for a stream with a single chunk `"abc"`
* `ReadableStream.from([..."abc"])` for a stream with three chunks `"a"`, `"b"`, `"c"`

Supersedes https://github.com/whatwg/streams/pull/1310. (Thanks to @lucacasonato for getting this started!)

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### :boom: Error: 422 Unprocessable Entity :boom: ###

[PR Preview](https://github.com/tobie/pr-preview#pr-preview) failed to build. _(Last tried on May 1, 2026, 1:42 PM UTC)_.

<details>
<summary>More</summary>


PR Preview relies on a number of web services to run. There seems to be an issue with the following one:

:rotating_light: [Spec Generator](https://www.w3.org/publications/spec-generator/) - Spec Generator is the web service used to build bikeshed/ReSpec specs

:link: [Related URL](https://www.w3.org/publications/spec-generator/?type=bikeshed-spec&output=html&url=https%3A%2F%2Fraw.githubusercontent.com%2FMattiasBuelens%2Fstreams%2F778c2370205364348ae3c59e3c0e84463577d4e3%2Findex.bs&force=1&md-status=LS-PR&md-Text-Macro=PR-NUMBER%201372)

**Error output:**

```json
[
    {
        "lineNum": "487",
        "messageType": "fatal",
        "text": "skipped: \"static ReadableStream from(async_sequence<any> asyncIterable)\""
    },
    {
        "lineNum": null,
        "messageType": "failure",
        "text": "Did not generate, due to errors exceeding the allowed error level."
    }
]
```

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</details>

You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/streams/pull/1372

-- Commit Summary --

  * Use `async iterable` webidl type in ReadableStream.from
  * Wire through reason
  * re-add comment
  * review comment
  * remove export
  * Merge branch 'main' into rs-from-async-sequence
  * Switch to `async_sequence`
  * Update reference implementation
  * Update spec text

-- File Changes --

    M index.bs (41)
    M reference-implementation/lib/ReadableStream.webidl (2)
    M reference-implementation/lib/abstract-ops/readable-streams.js (36)
    M reference-implementation/package.json (2)

-- Patch Links --

https://github.com/whatwg/streams/pull/1372.patchhttps://github.com/whatwg/streams/pull/1372.diff
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