TimeLib.h updated for use RTC circuit embedded with megaavr

Just for information,

I've updated original TimeLib.h library for use with internal RTC clock of Arduino Uno WiFi rev 2
https://github.com/goodchip/Time

Examples here: https://github.com/goodchip/Time/tree/master/examples/TimeInternalRTC

**Features:**

- use internal RTC circuit for real clock in board (not based on millis() function)
- additional attach/detach interrupt function to run user function at adjustable freq. synchronized by RTC
- adjustable top syncho between RTC and real second when setTime is called
- 100% compatible with 328p code (run without RTC) if interrupt are not used

**Current limitation:**

- use only external quartz for the moment.
- alpha version

_**Have fun!**_

Sylvain

Start by reading TimeLib.h and examples/TimeInternalRTC in the linked repository, then compare the RTC, interrupt, and setTime behavior with this core. The issue has no requested file change, test, or acceptance criteria, so completion is not defined.

Written by the indexing model from the issue text.