Move some position-related functions to PositionHelper.

Renames helper Entity.js to Position.js and moves to there:

• EntitiesNearPoint (from Attacking.js).
• InterpolatedLocation (from Attacking.js).
• TestCollision (from Attacking.js).
• PredictTimeToTarget (from Attack.js).

Also adds a test for the helper.

Differential Revision: D2940
Reviewed By: @wraitii
Comments by: @Stan, @vladislavbelov