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.

明示的な operator== と operator!= のペアを定義している node-addon-api の型を特定し、続いて順序比較について Maybe を調べます。冗長な != オーバーロードを削除し、比較動作を変更せずに、対象となる == または <=> 演算子を default 定義できれば完了です。