Position instances still support array access fallback

[Hamms]

For backwards compatibility during the transition from [x, y] to new Position(x, y).

As of https://github.com/code-dot-org/craft/pull/453, we should be able to exclusively use .x and .y rather than [0] and [1] to reference position values, so we can

• [ ] Update static methods like Position.add to use .x rather than [0] access option
• [ ] Remove [0] access option