Based on the example file from the announcement blog post http://sketchplugins.com/d/87-new-file-format-in-sketch-43
type UUID = string // with UUID v4 format
type SketchPositionString = string // '{0.5, 0.67135115527602085}'
type SketchNestedPositionString = string // '{{0, 0}, {75.5, 15}}'