Split item-list.module.css out to its own library

CSS is added on https://drupal-dev.ddev.site/admin/reports/fields but doesn't actually work, maybe a weight issue?

Found the problem - claro wants to extend, not override, the library.

Fixed a merge conflict but seems performance tests are failing.

Updated the performance tests, but the new layout builder failures are consistent so something is going on here.

Should be green now. Looked like very complicated issues in LayoutBuilderUITest but was in fact a typo in the starterkit theme library definition.

It is green now, and we're decreasing the page weight, that's great.
I am not sure about moving the file though, people are not supposed to reference the file by path, but it seems like it increases the likelihood this will be a breaking change?

@borisson_ the moved_files definition supports libraries_override explicitly - it transparently switches the override and triggers a deprecation error. That leaves two situations:

1. Someone modifying the file in hook_library_info_alter() - but data structures passed to alter hooks are excluded from backwards compatibility for this reason, we'd have the same problem changing the location in a major release too.

2. Someone directly referencing the file in a separate library definition, this feels very, very unlikely because until this MR it was loaded on every request anyway, so it would result in double-loading.

We've also had 📌 Move system/base component CSS to respective libraries where they exist Fixed in core since around 10.2 or 10.3, and not had any regressions reported that I'm aware of (this is one of many follow-ups to that issue to try to finish the meta).

Thanks for that information. Moving to rtbc based on #10.

The Needs Review Queue Bot → tested this issue. It no longer applies to Drupal core. Therefore, this issue status is now "Needs work".

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Rebased