[whatwg/streams] Add explainer for resizable buffers for BYOB readers (PR #1360)

I'm proposing we add support for resizable `ArrayBuffer`s to readable byte streams (as suggested in #1248). More specifically, developers should be able to pass an `ArrayBufferView` backed by a resizable `ArrayBuffer` to `ReadableStreamBYOBReader.read(view)`, and have the returned view still be backed by that same (resizable) buffer.

I expect the actual spec change to be fairly small:
* In the WebIDL, we'll add `[AllowResizable]` to the `view` argument to indicate that views can now be backed by both resizable and non-resizable buffers.
* In the specification, we'll need to make sure that we preserve the "resizability" of the backing buffer whenever we need to transfer it.

Expect a draft PR soon, so we can talk about implementation details. 😉

Note that supporting growable `SharedArrayBuffer`s is a non-goal of *this* proposal, but I still intend to open a separate proposal to support `SharedArrayBuffer`s. Then we can finally support reading directly into a (shared) WebAssembly memory. 😁

Open questions:
* Should the underlying byte source be allowed to resize `ReadableStreamBYOBRequest.view.buffer`?
  * I'm leaning towards "no", since we don't allow responding with more bytes than requested by `byobRequest.view.byteLength`.
  * This can be checked in `byobRequest.respond()` and `.respondWithNewView()`.
* Should we also support resizable buffers for [`autoAllocateChunkSize`](https://streams.spec.whatwg.org/#dom-underlyingsource-autoallocatechunksize)?
  * This would allow byte sinks to use memory more efficiently when they are used with a default reader. But are there practical use cases for this?
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/streams/pull/1360

-- Commit Summary --

  * Explain resizable buffers for BYOB readers
  * Add end-user benefits and alternatives

-- File Changes --

    A explainers/resizable-buffers-for-byob.md (120)

-- Patch Links --

https://github.com/whatwg/streams/pull/1360.patch
https://github.com/whatwg/streams/pull/1360.diff

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