[dave-365]

Fixes #1747

Description

This PR introduces C++20 std::span overloads for buffer-like types (Buffer<T>, ArrayBuffer, and TypedArrayOf<T>), allowing developers to pass bounds-carrying handles directly without manually managing separate raw pointers and length parameters:

• Added Buffer<T>::New, Buffer<T>::NewOrCopy, and Buffer<T>::Copy overloads accepting std::span.
• Added ArrayBuffer::New overloads accepting std::span.
• Added .Span() accessors to Buffer<T>, ArrayBuffer, and TypedArrayOf<T>.
• All std::span APIs are conditionally compiled under #if defined(__cpp_lib_span).
• Completely preserves ABI compatibility and backward compatibility for C++ < 20.
• Added comprehensive unit tests in test/buffer.cc and test/array_buffer.cc.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.75%. Comparing base (25c6b21) to head (1803c34).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1760      +/-   ##
==========================================
+ Coverage   63.68%   63.75%   +0.07%     
==========================================
  Files           3        3              
  Lines        2065     2072       +7     
  Branches      731      733       +2     
==========================================
+ Hits         1315     1321       +6     
  Misses        162      162              
- Partials      588      589       +1     

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