Prework Milestone 3: Adding player movement

[camunity]

Problem:
I want to add the ability to move my avatar using the arrow keys.

Implementation Suggestions:
This is blocked first by #8 and #9 so we should handle those first

Holding down the arrow keys should move the player continuously, and don’t rate limit the move commands.

Testing Criteria:
Open index.html in Chrome and confirm that we can send one move command per key down event.