Add new @primer/brand-css package

[rezrah]

Summary

Closes https://github.com/github/primer/issues/3658

List of notable changes:

What should reviewers focus on?

Steps to test:

Supporting resources (related issues, external links, etc):

Contributor checklist:

• [ ] All new and existing CI checks pass
• [ ] Tests prove that the feature works and covers both happy and unhappy paths
• [ ] Any drop in coverage, breaking changes or regressions have been documented above
• [ ] UI Changes contain new visual snapshots (generated by adding update snapshots label to the PR)
• [ ] All developer debugging and non-functional logging has been removed
• [ ] Related issues have been referenced in the PR description

Reviewer checklist:

• [ ] Check that pull request and proposed changes adhere to our contribution guidelines and code of conduct
• [ ] Check that tests prove the feature works and covers both happy and unhappy paths
• [ ] Check that there aren't other open Pull Requests for the same update/change

Screenshots:

Please try to provide before and after screenshots or videos

Before	After

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 8f81fd78443169e068f65a75d1f9e1dc22c9ff64

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 8 packages

Name	Type
@primer/brand-css	Minor
@primer/react-brand	Minor
@primer/brand-primitives	Minor
@primer/brand-e2e	Minor
@primer/brand-fonts	Minor
@primer/brand-config	Minor
@primer/brand-docs	Minor
@primer/brand-storybook	Minor

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Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions[bot]]

🟢 No design token changes found

[github-actions[bot]]

🟢 No visual differences found

Our visual comparison tests did not find any differences in the UI.