Add ATmega_Slow_PWM Library (#1744)

### Initial Releases v1.0.0

1. Initial coding for AVR **ATmega164(A/P), ATmega324(A/P/PA/PB), ATmega644(A/P), ATmega1284(P)** using [MightyCore](https://github.com/MCUdude/MightyCore)
2. The hybrid ISR-based PWM channels can generate from very low (much less than 1Hz) to highest PWM frequencies up to 500Hz with acceptable accuracy.

Author: khoih-prog

repositories.txt

@@ -5132,3 +5132,4 @@ https://github.com/natnqweb/EventSystem
 https://github.com/DIYODEmag/DIYsplay
 https://github.com/thomasfredericks/MicroOsc
 https://github.com/khoih-prog/ATmega_TimerInterrupt
+https://github.com/khoih-prog/ATmega_Slow_PWM