Previously we attempted to add a diff streaming animation to the AI composer helper: #1332, but it resulted in issues despite attempted fixes (#1338) so we temporarily suppressed the diff animation (#1341).

This update makes a second attempt at implementing the diff streaming animation. Instead of creating a custom diff algorithm, we make use of a third-party library [`jsDiff`](https://github.com/kpdecker/jsdiff) (which we added to core here: discourse/discourse#32833). While streaming, the diff animation often struggles with markdown links and images, so we make use of `markdown-it` parser to detect those cases and prevent breaking the animation.

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Co-authored-by: Sam Saffron <sam.saffron@gmail.com>