Bug: trigger_gravity and trigger_vphysics_motion use different vectors for their directional gravity keyvalues

[Klunkyone]

Describe the bug

For trigger_vphysics_motion, the default directional gravity keyvalue of 0 0 0 translates to downward gravity, whereas 90 0 0 (correctly) signifies a downward gravity vector for trigger_gravity and follows conventional rules for pitch, yaw, roll that all entities adhere to.

A new keyvalue for trigger_vphysics_motion should be created that represents an actual gravity vector rather than a transformation of the gravity direction. This keyvalue should be the same one used for trigger_gravity for parity.

To Reproduce

1. Create trigger_vphysics_motion with all default properties, check flag for physics objects
2. Default downward gravity vector is 0 0 0
3. Test ingame, gravity points down (Nooo! 0 0 0 isn't down!!)
4. Reflect on your mistakes and push a fix for this issue to staging immediately

Issue Map

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Expected Behavior

Using 90 0 0 as a directional gravity vector on trigger_vphysics_motion should translate to a downward gravity vector

Operating System

No response