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...

Start by locating the HardwareSerial API and its buffer handling in the ArduinoCore-API code, then review issues arduino/Arduino#2292 and arduino/Arduino#2291 for prior approaches. Implement and document the proposed guarded 9-bit path, preserving normal users' performance and size, and verify the affected Serial behavior.

Written by the indexing model from the issue text.