Adopt C++20 defaulted comparisons (operator== / <=>)

Several node-addon-api types define operator== and operator!= as an explicit pair, where the != is always return !operator==(other);. In C++20 the compiler synthesizes a != b from operator==, so the hand-written != overloads become redundant and can be removed. Where the comparison is plain memberwise equality, operator== itself can be = defaulted.

Regarding <=>, the only plausible type is Maybe<T> where T is ordered.

定位定义了显式 operator== 和 operator!= 对的 node-addon-api 类型，然后检查 Maybe 是否存在有序比较。完成的标准是移除冗余的 != 重载，并将符合条件的 == 或 <=> 运算符设为 default，同时不改变比较行为。