Upgrade to eslint v9

[rezrah]

Summary

Towards https://github.com/github/primer/issues/5926

Upgrades our ESLint toolchain to v9, which includes a number of breaking changes.

This PR looks to minimize the blast zone of changes to only what's necessary. Follow up PR's will be required to upgrade some adjacent ESLint plugins.

For benchmarking purposes... ESLint (v8) vs ESLint (v9):

Tool	Lint Time	Format Time	Total Time	Source
Before: ESLint (v8)	53s	13s	66s	CI Run
After: ESLint (v9)	54s	13s	67s	CI Run
After: BiomeJS	1s	1s	2s	CI Run

Includes BiomeJS as an experiment only. More on that another time.

List of notable changes:

• Manually upgraded ESLint from v8 to v9
• Updated only the ESLint plugins that were needed for v9, not those that continue to work
  • This is to limit size of this PR and not add 'new rules' to the mix
  • Some rule changes in affected packages, neccessitated adjustments to the code. No functional changes expected, but a changeset has been included for transparency.
  • The biggest change is that _ prefixing of component names is now deemed an error by rules-of-hooks. All instances replaced.
• Enabled linting in Next.js .mdx files, which previously wasn't on
• Vendored i18n plugin, as it's now turned off in github/recommended but we still need it in Brand

What should reviewers focus on?

• Ensure CI is passing

Steps to test:

1. Go to files changed and review chages, they should track with the updates mentioned above.
2. Comment on anything that looks incorrect or strange

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

[changeset-bot]

🦋 Changeset detected

Latest commit: 3bd3aeb

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

This PR includes changesets to release 8 packages

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

Not sure what this means? Click here to learn what changesets are.

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

[github-actions]

🟢 No design token changes found

[github-actions]

🟢 Unit test coverage changes found

Unit test coverage has been updated through this PR.

Changes: 0 new tests, 0 removed tests, 0 improved, 2 decreased

Component/Hook	Statements	Functions	Branches	Change
Bento	83.1% 82.4%	94.4%	75.2% 74.8%	-0.8%
AnchorNav	93.0%	91.7%	78.6% 78.1%	-0.4%

[github-actions]

🟢 No visual differences found

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

[Copilot]

Pull Request Overview

This PR migrates the project from ESLint v8 to ESLint v9 with flat config support. The main changes include:

• Replacing .eslintrc.js files with eslint.config.mjs flat config files
• Upgrading ESLint and related plugins to v9-compatible versions
• Converting CommonJS imports to ESM imports (e.g., import clsx from 'clsx' → import {clsx} from 'clsx')
• Vendoring the i18n-text/no-en rule due to ESLint v9 incompatibility
• Refactoring internal component function names for consistency
• Migrating the Playwright test generator script to ESM

Reviewed Changes

Copilot reviewed 186 out of 188 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
eslint.config.mjs	New flat config replacing root .eslintrc.js with vendored i18n rule integration
packages/repo-configs/custom-lint-rules/no-english-text.js	Vendored i18n rule for ESLint v9 compatibility
packages/react/src/**/*.tsx	Updated clsx imports and component naming conventions
packages/e2e/scripts/playwright/*.ts	Migrated test generator from CommonJS to ESM
package.json	Upgraded ESLint and plugins to v9-compatible versions
.eslintrc.js (deleted)	Removed legacy config files across all packages

[danielguillan]

This looks great! No issues detected when running locally.