Rollup merge of #89535 - notriddle:notriddle/error-index-generator-js, r=Mark-Simulacrum

fix busted JavaScript in error index generator

The old JavaScript didn't work. It filled the browser console with "e.previousElementSibling not defined" errors, because it didn't account for the example-wrap div that a newer version of rustdoc added.

Additionally, it had copied versions of utility functions that had been optimized in rustdoc main.js. This version updates those.