feat: add CSS unit baseline checking

[ryo-manba]

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?

Add baseline checking for CSS units (svh, dvh, cqw, etc.) to the use-baseline rule.

What changes did you make? (Give an overview)

• Extract unit data from web-features package via css.types.length.* BCD keys in tools/generate-baseline.js
• Add unit checking logic to src/rules/use-baseline.js (Dimension node handling, @supports guard support)
• Add allowUnits option to suppress specific unit warnings

Related Issues

fixes #402

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