refactor: update build config for @primer/react and rename import CSS plugin for Rolldown

[joshblack]

Updated build tooling for @primer/react to use Rolldown, including renaming the internal CSS import plugin package to match Rolldown naming.

Changelog

New

N/A

Changed

• Updated @primer/react to build JavaScript with Rolldown.
• Renamed rollup-plugin-import-css to rolldown-plugin-import-css.
• Updated the import CSS plugin package to export its TypeScript source directly for Rolldown consumption.
• Updated the export size helper and Dependabot bundler grouping for Rolldown.

Removed

• Removed the package Rollup build config and direct Rollup build dependencies.
• Removed the build step and Rollup build config for rolldown-plugin-import-css.

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; build tooling-only change with no public runtime API change

Testing & Reviewing

Validated with npm run build, npm run type-check, npm run lint, npm run lint:css, npm run format:diff, and npm test -- --run.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github-ui (Learn more about how to run integration tests)

[changeset-bot]

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Latest commit: 6b895e7

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[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. If this doesn't work, you can also use the original workflow here. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.

To publish a canary release for integration testing, apply the Canary Release label to this PR.

[primer]

🤖 Lint issues have been automatically fixed and committed to this PR.

[primer]

🤖 Lint issues have been automatically fixed and committed to this PR.