fix: migrate baseline generation to @webref/css

[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 migrates baseline generation away from the deprecated mdn-data package and over to @webref/css.

What changes did you make? (Give an overview)

I updated tools/generate-baseline.js to use @webref/css as the source of known CSS function names. I also replaced mdn-data with @webref/css in package.json, updated the update-baseline workflow to refresh @webref/css instead of mdn-data, and regenerated the baseline data.

During the migration, I also handled the string() edge case so we do not incorrectly treat css.types.string support as support for the string() function.

Related Issues

Closes #376

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

[DMartens]

Changes LGTM, thanks.
Can you please also add a test case for string for use-baseline?

[nzakas]

LGTM pending @DMartens's suggestion.