Add two new plugins

[gcormier]

Two new plugins under development

1. grblhal-thermistor - https://github.com/gcormier/grblhal-thermistor
   Read temperature of spindle via thermistor and analog input

2. grblhal-pneumaseal - https://github.com/gcormier/grblhal-pneumaseal
   Intelligently manage air seals on spindle so we don't waste air all the time.

Not sure if you want to wait to merge these or not? I can develop locally first if that's better.

[terjeio]

I prefer to add links to your repos and make them available from the Web Builder as 3rd party plugins - so that I do not have to maintain them.

To make the plugins compatible with the RP2040 and ESP32 drivers you have to add a CMakeLists.txt to each of them - similar to this.

[gcormier]

Is there a web builder repo I can submit PR's to?

I will add cmake files.