Evolve approach to reference files ☂️
The following tasks are involved in evolving our approach to reference files.

- [x] Move reference files to their own repo (https://github.com/sketch-hq/sketch-reference-files)
- [x] Improve the automation so its feasible to generate output for multiple versions of Sketch (e.g. able to retrospectively add a new ref file type to the output) (https://github.com/sketch-hq/sketch-reference-files)
- [x] Publish ref files to npm so they can be imported by other projects. Module should export the file JSON (https://www.npmjs.com/package/@sketch-hq/sketch-reference-files)
- [X] Add ref files for some missing use cases (slice, blurs, export presets, smart layout, open type) (https://github.com/sketch-hq/sketch-reference-files/commit/2e8408a0ea5f99507f6dceeb8b1334cb42040aae)