[Grbl_Esp32 Issue#293] Extra Settings

I am playing with a new machine that needs to store some additional settings. I decided to add some generic values that can be used.

– I added $80 through $84 for signed int16 values
– I added $90 through $94 for float values.

These can be used to store values that a special machine needs. These could be calibration values, kinematic stuff, etc. Grbl will manage the storage, modification and reporting of the values. They can be accessed from any custom code you would add.

If you are working of a new feature like PID motor control, etc. Use these settings. If the feature evolves enough to be part of the main code, we can find a permanent home for them in the lower numbers.