Add `entity:isMoving()` and `entity:isFalling()` to EntityAPI

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[!NOTE] This is an implementation of a feature suggested in the Discord server.

This PR adds the following functions to EntityAPI:

• entity:isMoving() - Returns true if entity's velocity in any coordinate is not zero, aka when entity is moving in any direction.
• entity:isFalling() - Returns true if entity's velocity in Y coordinate is below zero AND entity is not on the ground.