[FeatureRequest] Add M303 cancelling to M108

[droftarts]

Currently, nothing can cancel M303 heater tuning except emergency stop. Add cancelling M303 to M108.
May be related to #765 (Allow aborting macros Including homing moves - "not quite an emergency, but STOP")

Forum discussions:
https://forum.duet3d.com/topic/35735/m0-still-not-working-to-cancel-heater-pid-tuning
https://forum.duet3d.com/topic/30848/unexpected-behaviour-3-5-0b1

Slack conversation:
@dc42: If we are going to allow it to be cancelled, it would make more sense to use M108 for that purpose. Meanwhile, use estop. It's not really the same as that other issue (which itself covers a few different things) (#765).