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.

首先定位现有的 Data() 和长度访问器这一对接口，然后查找接受指针/长度输入的 New 和 Copy 工厂。检查项目如何处理 C++ 版本保护以及现有的重载。完成的标准是以增量方式在两个方向上支持 std::span，同时保留指针/长度重载以确保 ABI 兼容性。