[whatwg/streams] Keep partial bytes in the queue when closing a byte stream (PR #1389)

When a readable byte stream is closed while the first pending pull-into descriptor holds only part of an element, `ReadableByteStreamControllerClose` used to error the stream and throw. Its callers in "close a ReadableStream" and `ReadableByteStreamTee` use `!`, so that throw was not allowed.
 
Instead, the stream now becomes close-requested. When the underlying byte source responds to the pending BYOB request, the incomplete element is put back into the queue, and the pending read-into requests are handled as if they were made after `close()`: each one is fulfilled with as many whole elements as are available, or fails with a `TypeError` if not even one element is available. The stream becomes closed once the queue is drained. Similarly, a `read(view)` on a close-requested stream whose queue cannot reach the minimum fill no longer errors the stream.
 
Fixes #1387. Alternative to #1388.

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   * https://github.com/web-platform-tests/wpt/pull/64155
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You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/streams/pull/1389

-- Commit Summary --

  * Keep partial bytes in the queue when closing a byte stream
  * Reuse existing abstract ops for close-requested byte streams
  * Clarify note in ReadableByteStreamControllerCommitPullIntoDescriptor

-- File Changes --

    M index.bs (127)
    M reference-implementation/lib/abstract-ops/readable-streams.js (107)

-- Patch Links --

https://github.com/whatwg/streams/pull/1389.patch
https://github.com/whatwg/streams/pull/1389.diff

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