Replace MPLAB-X build system with CMake

[bessman]

This works already. Just needs a little bit of clean up and testing before merge.

Using MPLAB for building has several drawbacks:

• It's proprietary
• It forces developers to use MPLAB instead of their IDE of choice
• The github build workflow has to install MPLAB every time it runs.

[bessman]

I added a search path for MacOS in https://github.com/fossasia/cmake-microchip/pull/1. Could you check if it works without manually setting MICROCHIP_XC16_PATH, please?