fix: sync with upstream csstree v3.2.1

[xbinaryx]

Prerequisites checklist

• I have read the contributing guidelines.

AI acknowledgment

• I did not use AI to generate this PR.
• (If the above is not checked) I have reviewed the AI-generated content before submitting.

What is the purpose of this pull request?

This PR updates @eslint/csstree to match upstream csstree.

Related Issues

Closes #118

Is there anything you'd like reviewers to focus on?

[nzakas]

LGTM. Thanks for your hard work on this!

[nzakas]

Would like @DMartens and @lumirlumir to review before merging.

[DMartens]

Changes LGTM, thanks.

[lumirlumir]

I think this PR needs to be merged using "Merge commits", not "Squash merge".

Based on this article and my understanding, squash merging turns the PR into a single commit SHA and causes the commit SHA information from the upstream csstree/csstree to be lost, so the "This branch is 106 commits ahead of and 23 commits behind csstree/csstree:master" information would not be resolved.

I'm not 100% sure, so it would be helpful to double-check the information above. But, if the description is correct, merging the commits edb0e59, 3c30ab1, and 99394e8 into a single commit would be helpful, since a "Merge commit" will retain those commits on the main branch as-is.

[nzakas]

@lumirlumir I think you're correct, I'm just not sure it's worth the trouble of squashing three commits and the merging the rest just to get the GitHub numbers to line up. I think as long as we have a commit that says "syncing with upstream csstree vx.y.z", that should be good enough for our needs.

[lumirlumir]

If it's acceptable not to squash the three commits into a single commit, I'm fine with that.

Changes LGTM. Please merge this PR once everything is ready, as I don't have permission to use "Merge commits".