Signed-off-by: Melissa Kilby <[email protected]>

* Add `AsyncBackpressuredStream` proposal and implementation

# Motivation

The pitch to add external backpressure support to the standard libraries `AsyncStream` got returned for revision since there are larger open questions around `AsyncSequence`. However, having external backpressure in a source asynchronous sequence is becoming more and more important.

# Modification

This PR adds a modified proposal and implementation that brings the Swift Evolution proposal over to Swift Async Algorithms.

* Update proposal and implementation

* Update proposal

* Add example project

* Formatting

* Fix Swift 6.0 build

* Future direction for ~Copyable elements

* Apply formatting

* Fix CI

* Move to 6.1 and update proposal

* Guard tests

* Minor edits to the proposal

* Fix revision order

* FIxup setOnTerminationCallback

* Address review feedback

* Rename to `MultiProducerSingleConsumerAsyncChannel`

* Allow one termination callback per source.

* Fix all sendable warnings

* Remove unbounded strategy, rename copy -> makeAdditionalSource, rename asyncSequence() -> elements(), and move enqueue and cancel to the `CallbackHandle` previously called `CallbackToken`

* Remove Example and fix docs and fix format

* fixes grammar and some word order issues

* Remove unnecessary consume

---------

Co-authored-by: Joe Heck <[email protected]>

….8 so that (apple#377)

symbols that are only available in later versions (for example, MultiProducerSingleConsumerAsyncChannel)
are reflected in the hosted documentation.

…le#380)

`nonisolated(nonsending)` is a 6.2 language feature that allows the inheritance of the callers isolation. The new `MPSCAsyncChannel.Source` send methods should adopt this to avoid unnecessary isolation hops.