Define the property to signal abort to UnderlyingSink even when it has a pending write or close operation. https://github.com/whatwg/streams/issues/1015
<rdar://problem/79134522>
FYI: we have changed the API here slightly to no longer include an abortReason property; instead the reason will be exposed on the signal itself, per https://github.com/whatwg/dom/pull/1027.