Feature: Folinn BD600 class in "Spindles" for RS485 support

[apopanda]

Machine Context

Running a Root controller with the G-Penny spindle + VFD setup. They sent a Folinn BD600. It's not supported via RS485, so I'll write up the support

Feature Description

I'm attempting to create a working setup for this VFD for my use. After I get it working (and the entire process is followed), it could be merged. PR guidelines state, that an issue needs to be made for discussion. The code will be here:
https://github.com/apopanda/FluidNC
Manual for the VFD is here
https://cononmotor.com.au/wp-content/uploads/2017/09/BD600-Manual.pdf
And contains most of the necessary addresses, responses, etc.

Other Approaches

I really don't want to use PWM or switch cables around, so I'd rather implement it.

How I Can Help

There's no need for help from the Developers right now. I'm writing the implementation myself.

[MitchBradley]

I am starting to shy away from including more VFDs into the code base. Every one increases our support cost.