Enforce style containment to <Steps> list items

[HiDeoo]

Description

This pull request enforces style containment to the <Steps> component list items.

Even tho this change was not technically required until now, Chrome 126 having an Early Stable release date sets to Wed, Jun 5, 2024 breaks the current implementation.

Any <Steps> component containing a nested list items (Markdown/HTML list, tabs, file tree, etc.) will have its counter broken. Here is an example from the Starlight documentation with this change:

The change is probably part of Chrome re-write of CSS counters and ordered list numbering code but it's a bit difficult to figure out if the behavior is a bug or a proper implementation of the spec considering the whatwg repository contains many issues related to this topic, some efforts have been done to clarify the behavior but it's still a bit unclear and there seems to be some exceptions when the default counter (the one we use) is read from a ::before pseudo element.

I'll open an issue in the Chromium tracker altho, even if it ends up being a bug, a fix will probably take a while to be released (Wed, Aug 14, 2024 at the earliest) so I think it's better to enforce the style containment now considering it's the behavior we want.

Note that to reproduce the issue, you need at least Chrome Beta (126 at the time of the PR).

[changeset-bot]

🦋 Changeset detected

Latest commit: e132b8b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@astrojs/starlight	Patch

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Name	Status	Preview	Updated (UTC)
starlight	✅ Ready (Inspect)	Visit Preview	Jun 4, 2024 1:57pm

[delucis]

Thank you for flagging this early and finding the fix @HiDeoo! 💖