Support for Sketch format

Sketch (not to be confused with Skencil, which was formerly known by the same name) is a popular MacOS program for illustration and design. It has an open file format (MIT license) which has already been implemented in a number of viewers and at least one other graphics program. It would be great to at least be able to import Sketch files into Inkscape the way we can with AI or PDF.

- SketchSVG is an example open source project that extracts vectors from Sketch files: https://github.com/eBay/SketchSVG
- Avocode is a Linux-compatible design program that supports the format: https://avocode.com/open-sketch-on-linux
- Descriptions of the format are available here: https://developer.sketch.com/file-format/
- As well as on GitHub: https://github.com/sketch-hq/sketch-file-format
- Here is a set of test files: https://github.com/sketch-hq/sketch-reference-files
- A third party JS library they recommend: https://github.com/Lona/sketch-file