Sketch API Parity

Go through the models in the Sketch plugin API and get everything up to somewhat parity.
https://developer.sketch.com/reference/api/

The output of this should be other issues of where these models fall short of the Sketch plugin API.

Start by reviewing the models in the Sketch plugin API reference at developer.sketch.com/reference/api/ and comparing them with the models in this repository. Record each parity gap as a separate follow-up issue; done means the gaps are documented as actionable issues rather than implementing the full parity effort in this issue.

Written by the indexing model from the issue text.