Full official support for the new ATmega328PB

The ATmega328P microcontroller, the chip that raised the origin of Arduino (Diecemila, Duemilanove, UNO, Nano, Mini, BT, Fio, LilyPad, Pro...), also in different kits and numerous community boards, is marked as **NRND** by the manufacturer. This means that it will be retired in favor of its replacement, the ATmega328PB, with new features.

This brings several consequences:
- When this happens, all boards using this chip will have to be retired, ending an entire era.
- Without official support for the new chip, all new developments made now cannot be done with the new chip, compromising its maintenance and product life, or having to look for alternatives in other microcontrollers.
- No other Arduino product has the robustness, reliability and library support of this chip, being the heart of probably millions of products worldwide. Without support for the new chip all of this would be lost.
- The community has adopted Arduino as the de facto standard for pin numbering, peripheral naming, etc. Without official support different projects can be created with conflicting definitions between them.
- The community follows in the wake created by this chip and in many cases not taking advantage of that leadership is abandoning them.
- In a way, it is a moral debt to the community.
- Supporting the new chip is a lovely way to close a legacy :heart: .

So, please, consider full support (covering new features and new bootloader) to the new chip. May we continue to enjoy Arduino for many more years to come.

No files, tests, or entry points are named in the issue. Start by scoping the ATmega328PB feature and bootloader requirements within ArduinoCore-avr; done would mean official support for the new chip, including its new features and bootloader.

Written by the indexing model from the issue text.