feat(async/unstable): add Channel

[tomas-zijdemans]

Adds Channel<T> to @std/async (as the concurrency module was rejected).

It is a typed async channel for passing values between concurrent tasks. It supports bounded buffering with backpressure, unbuffered rendezvous mode, non-blocking trySend/tryReceive variants, async iteration, and clean shutdown via close() with optional error propagation. The channel implements both Disposable and AsyncDisposable for use with using and await using.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.89%. Comparing base (48bd522) to head (5a13f54).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7035      +/-   ##
==========================================
+ Coverage   93.88%   93.89%   +0.01%     
==========================================
  Files         627      628       +1     
  Lines       49995    50136     +141     
  Branches     8794     8825      +31     
==========================================
+ Hits        46936    47077     +141     
  Misses       2478     2478              
  Partials      581      581              

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[kt3k]

Looks an interesting idea. Thanks for the suggestion. LGTM