feat(async/unstable): add AbortSignal and toReadableStream to Channel

[tomas-zijdemans]

• AbortSignal support for send() and receive()
• Cancellation-aware internal queues
• Improved tryReceive() return type
• toReadableStream() method
• close() overload signatures
• Minor optimizations

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.42%. Comparing base (1cd63ca) to head (a376120).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7086      +/-   ##
==========================================
+ Coverage   94.41%   94.42%   +0.01%     
==========================================
  Files         630      630              
  Lines       50490    50569      +79     
  Branches     8949     8972      +23     
==========================================
+ Hits        47669    47749      +80     
  Misses       2249     2249              
+ Partials      572      571       -1     

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