Add 9bit support to HardwareSerial

I stumbled over a few different implementations / tweaks how Arduino could read 9bit data using Serial objects; the core library does not support 9bit, while the standard UART HW does ...

I am aware of issues arduino/Arduino#2292 and arduino/Arduino#2291 - however I'd like to take a differen timplementation path: use guard macro to conditionally enable the extra code (and twice as long buffer). This way, normal user will not see any performance/size penalty, but developer who needs 9bit support could enable it at the documented cost.

Diffs will be attached shortly as PR...

首先在 ArduinoCore-API 代码中定位 HardwareSerial API 及其缓冲区处理，然后查看 issue arduino/Arduino#2292 和 arduino/Arduino#2291，了解之前的处理方案。实现并记录所提议的受保护 9 位路径，在保持普通用户的性能和大小不变的同时，验证受影响的 Serial 行为。

由索引模型根据 Issue 内容生成。