v1.2.0 to add support to MegaCoreX core

### Releases v1.2.0

1. Fix `multiple-definitions` linker error. Drop `src_cpp` and `src_h` directories
2. Add support to `MegaCoreX` core, including ATmega4809, ATmega4808, ATmega3209, ATmega3208, ATmega1609, ATmega1608, ATmega809 and ATmega808
3. Improve accuracy by using `float`, instead of `uint32_t` for `dutycycle`. Check [Change Duty Cycle #1](khoih-prog/ESP8266_PWM#1 (comment))
4. DutyCycle to be optionally updated at the end current PWM period instead of immediately. Check [DutyCycle to be updated at the end current PWM period #2](khoih-prog/ESP8266_PWM#2)
5. Optimize library code by using `reference-passing` instead of `value-passing`
6. Add example [multiFileProject](examples/multiFileProject) to demo for multiple-file project
7. Update examples accordingly

Author: khoih-prog

library.json

@@ -1,8 +1,8 @@
 {
   "name": "megaAVR_Slow_PWM",
   "version": "1.2.0",
-  "keywords": "timer, interrupt, hardware, isr, isr-based, pwm, isr-based-pwm, timing, control, device, hardware-timer, mission-critical, accuracy, precise, megaavr, mega-avr, atmega4809, nano-every, uno-wifi-rev2",
-  "description": "This library enables you to use ISR-based PWM channels on an Arduino megaAVR board, such as UNO WiFi Rev2, AVR_Nano_Every, etc., to create and output PWM any GPIO pin. It now supports 16 ISR-based PWM channels, while consuming only 1 Hardware Timer. PWM channel interval can be very long (ulong microsecs / millisecs). The most important feature is they're ISR-based PWM channels, supporting lower PWM frequencies with suitable accuracy. Their executions are not blocked by bad-behaving functions or tasks. This important feature is absolutely necessary for mission-critical tasks. These ISR-based PWMs, still work even if other software functions are blocking. Moreover, they are much more precise (certainly depending on clock frequency accuracy) than other software-based PWM using millis() or micros(). That's necessary if you need to control devices requiring high precision. Now you can change the PWM settings on-the-fly.  Now supporting MegaCoreX.",
+  "keywords": "timer, interrupt, hardware, isr, isr-based, pwm, isr-based-pwm, timing, control, device, hardware-timer, mission-critical, accuracy, precise, megaavr, mega-avr, atmega4809, nano-every, uno-wifi-rev2, megacorex",
+  "description": "This library enables you to use ISR-based PWM channels on an Arduino megaAVR board, such as UNO WiFi Rev2, AVR_Nano_Every, etc., to create and output PWM any GPIO pin. It now supports 16 ISR-based PWM channels, while consuming only 1 Hardware Timer. PWM channel interval can be very long (ulong microsecs / millisecs). The most important feature is they're ISR-based PWM channels, supporting lower PWM frequencies with suitable accuracy. Their executions are not blocked by bad-behaving functions or tasks. This important feature is absolutely necessary for mission-critical tasks. These ISR-based PWMs, still work even if other software functions are blocking. Moreover, they are much more precise (certainly depending on clock frequency accuracy) than other software-based PWM using millis() or micros(). That's necessary if you need to control devices requiring high precision. Now you can change the PWM settings on-the-fly. Now supporting MegaCoreX.",
   "authors":
   {
     "name":  "Khoi Hoang",