[dave-365]

Fixes #1748

Summary of Changes:

• Leverage C++20 defaulted comparisons (operator== and <=>) when compiling under C++20 or later.
• Remove redundant hand-written operator!= implementations in C++20 mode for Value, Reference<T>, Object::iterator, Object::const_iterator, and Maybe<T> as the compiler synthesizes them automatically from operator==.
• Adopt = default equality and three-way comparison (<=>) for BasicEnv.
• Add spaceship operator (<=>) for Maybe<T> using std::compare_three_way_result_t<T> to support relational ordering when T supports comparison.
• Preserve full backward compatibility for C++14/C++17 compilers under #if !defined(NAPI_HAS_CPP20_COMPARISONS).
• Add binding_cpp20 target and unit tests covering C++20 comparison behavior.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 63.75%. Comparing base (25c6b21) to head (0cce782).

Files with missing lines	Patch %	Lines
napi.h	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1761      +/-   ##
==========================================
+ Coverage   63.68%   63.75%   +0.07%     
==========================================
  Files           3        3              
  Lines        2065     2072       +7     
  Branches      731      737       +6     
==========================================
+ Hits         1315     1321       +6     
  Misses        162      162              
- Partials      588      589       +1     

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