Expose minimum car speed (deadzone speed) as LUA value

[Zanthra]

As reported here: https://mods.factorio.com/mod/GTTS/disc ... 5f5e40f212 and in discord some time ago, cars cannot move in reverse when the acceleration capabilites are adjusted to match 240+ UPS.

This is caused because there is a deadzone speed of 1.7 km/h under which the speed is set to 0. If the car cannot acceelerate to above that speed in a single tick, it won't move at all.

There does not seem to be any reference to minimum speed, deadzone speed, or some sort of static friction that might be adjustable to change this minimum speed to something lower. I expect it is a value still hidden in the code and inaccessible. It would be nice to have it available through LUA.