Rewrite millis() and micros() to enable chaning of the prescaler of Timer0

I like to use a different prescaler for Timer0 because I need 6 PWM outputs on a high frequency to control two stepper motors with microsteps.

I created a solution myself. The only thing I had to do was changing wiring.c to make calculations using the actual settings of the configuration registers instead of the defined default values.

See: https://github.com/GewoonGijs/platformio-pkg-framework-arduinoavr/blob/master/cores/arduino/wiring.c

Start with cores/arduino/wiring.c and compare the proposed approach in linked pull request #80 and the referenced solution. Check how Timer0 prescaler configuration affects millis() and micros(), then verify that timing calculations use the actual register settings while preserving existing default behavior.

Written by the indexing model from the issue text.