Improve performance for huge diffs in conan report diff HTML output

[AbrilRBS]

Changelog: Fix: Improve support for huge diffs in conan report diff HTML output
Docs: conan-io/docs#4279

My weekend project of learning about the Intersection Observer API

Still needs some tests, but this works great locally for big diffs that prior to this would struggle to work properly.

There is an issue with upwards scrolling unloaded content, but other than that, this looks almost ready

[perseoGI]

Tested locally with really huge diff files and I can confirm it works great.

Thank you, @AbrilRBS, for the big improvement!

Screen.Recording.2025-10-22.at.11.09.04.mov

[ErniGH]

LGTM. The difference is very noticeable with large diffs. These diffs for libraries with many changes between versions can reach up to 600,000 lines of code in the HTML, and this report diff manages to render the changes.