Make a test platform

It would be nice for us to have tests covering the code. Currently, any change has the ability to cause a multitude of undetected bugs, if we don't manually test well. Having defined behaviors would help prevent future rewrites like Arrangers or Masks 2.0 from breaking existing behaviors.

The general idea is that we would create a new `Platform` implementation that gives out test-friendly implementations of WorldEdit APIs. We could then write a multitude of integration tests _solely based on command input_ to allow us to detect when we break anything in any user-facing part of the API.