Implement text segmenting and bidirectional text

It would be great to improve the text behavior in the canvas to properly support bidirectional text, and text segmenting per script.

- [ ] Research which libraries exist on nmp that may help us
- [ ] Write a new Fontra .js module that provides a high level interface for text segmenting, and any functionality that we can't reuse from third-part modules.
- [ ] Refactor the line layout logic to support multiple text runs in a single line

Prior art: https://github.com/justvanrossum/fontgoggles/blob/master/Lib/fontgoggles/misc/segmenting.py