Type B system Post "gate" sensor

[53Aries]

Add implementation of pre and post gate sensor for type B systems. The pre gate sensor will mainly be used to auto load the lane, if the lane is empty and the sensor becomes triggered the lane extruder will activate to pull filament in until it reaches the post gate sensor. The post gate sensor will also be the filament parking position when unloading from the toolhead.
If implementing the pre gate sensor for auto load is too complex, that task can be handed off to an external macro and only the post gate sensor would be utilized by HH.

[moggieuk]

Typically, a post gate sensor would be shared and would be just after the point where the feeds are combined. Then the normal loading logic is retained of reaching the post-gate sensor and then "parking" a short distance away from it. Such a use case is already supported albeit, incompletely for type-B designs.

I have just picked up completing the missing pieces for type-B so can revisit this after the initial support is complete.