Investigate other JS diff libraries for character diffs

[danvk]

I'm using a personal port of https://github.com/cemerick/jsdifflib to TS (#185). It's 17 years old.

https://github.com/qiao/difflib.js is only 12 years old, is much more popular (500k weekly downloads) and has type declarations on DefinitelyTyped. And maybe it's faster?

This is the most-popular diff library: https://github.com/kpdecker/jsdiff (45M weekly downloads!). It also has types on DT. It's based on Myers, the same default diff algorithm a git.