test: cover revert-rule CSS-wide keyword

[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 is the purpose of this pull request?

This PR adds test coverage and documentation for the revert-rule CSS-wide keyword, which will be added to the lexer in eslint/csstree#151.

What changes did you make? (Give an overview)

• Added revert-rule test cases to font-family-fallbacks, relative-font-units and no-invalid-properties.
• Added revert-rule to the list of always-allowed global values in font-family-fallbacks.md and relative-font-units.md.

Related Issues

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

[DMartens]

Should we also add test case for:

• no-invalid-properties: seems to have no CSS-wide keyword tests
• use-baseline: some CSS-wide keywords have a baseline entry but revert-rule seems to be missing

[Pixel998]

no-invalid-properties: seems to have no CSS-wide keyword tests

This PR also includes tests for no-invalid-properties.

use-baseline: some CSS-wide keywords have a baseline entry but revert-rule seems to be missing

Opened #542.

[DMartens]

Changes LGTM, thanks. Waiting for the csstree PR to land.