Support C++20 span types

node-addon-api's buffer-like types pass their backing memory as a raw pointer plus a separate length, both on the way out (Data() + a length accessor) and on the way in (the New/Copy factories). Callers must keep the two values in sync manually, with no bounds information attached to the pointer.

Once C++20 is the minimum supported standard, we should adopt std::span in both directions. These additions are ABI-compatible (existing pointer/length overloads stay), and give callers a single bounds-carrying handle that works with range-based for, <algorithm>, and <ranges>.

This can be an additive change with a C++ version guard.

Comece localizando o par existente formado por Data() e pelo accessor de comprimento, depois as factories New e Copy que aceitam entradas de ponteiro/comprimento. Revise como o projeto lida com as verificações de versão do C++ e com os overloads existentes. O trabalho estará concluído quando houver suporte aditivo a std::span em ambas as direções, mantendo os overloads de ponteiro/comprimento por compatibilidade de ABI.