[Request] Add mappings for wheel throttle forward and reverse

[awdwyn]

Using a controller splits trigger inputs as two half-range axes. Mapping the wheel throttle axis to one of these only allows for forward motion from 0 to max speed.

Some possible ideas to fix this are:

• add a negative wheel throttle axis mapping
• add forward and back wheel movement button mappings (same as what W and S currently do)
• add a button mapping to invert wheel throttle axis input value (allowing the single axis to shift to reverse)

I would prefer the first option, as it is in line with how other axes are mapped with negatives.