feat: check CSS-wide keywords in use-baseline

[Pixel998]

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 did you do?

/* eslint css/use-baseline: ["error", { available: 2021 }] */

a {
  font-size: revert-layer;
}

What did you expect to happen?

The declaration to be reported. revert-layer only reached Baseline in 2022, so with { available: 2021 } it is not yet available.

What actually happened?

No error is reported.

What is the purpose of this pull request?

This PR makes use-baseline check CSS-wide keywords.

What changes did you make? (Give an overview)

• Added a globalKeywords map to the generated baseline data, sourced from the css.types.global_keywords.* compat keys.
• checkPropertyValueIdentifier now consults that map before the per-property propertyValues lookup.

Additional comments

revert-rule is not covered. web-features does not expose it yet, so there is no Baseline status to check against. The pattern added here already matches its compat key, so it will be picked up automatically.

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