Add anti-aliasing to Text and other components

[rezrah]

Summary

Closes #871
Part of https://github.com/github/primer/issues/4601
Follow up to #482

Adds targeted font-smooth to Text and other components in dark mode.

Important

Applying blanket AA on websites using font-smooth is NOT recommended due to missing web standards

It's also not recommended to use it anywhere except dark mode.

We will only apply AA when:

• User agent is webkit or macos based
• dark color mode is enabled
• Fonts are not using light or extralight variants, due to poor legibility
• Text size is above 16px

List of notable changes:

• Enabled AA on Text component, which cascades to most components that use it internally
• Manually added to components that don't use Text internally. E.g. Prose
• Added guardrail conditions for when to apply it
  • Includes tests and a storybook story

What should reviewers focus on?

• Use Chrome or Safari to view this page, or any story in dark color mode

Steps to test:

1. Storybook

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]

🦋 Changeset detected

Latest commit: 0a59583

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

This PR includes changesets to release 6 packages

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

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[github-actions]

🟢 No design token changes found

[github-actions]

🟢 No visual differences found

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

[jesussandreas]

Looks good to me, no concerns. Thank you for raising the accessibility concerns as well.