[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.