refactor: update build config for rollup-plugin-import-css to rolldown

[joshblack]

Closes #

N/A

Changelog

New

N/A

Changed

• Updated rollup-plugin-import-css to export its TypeScript source directly for Rolldown consumption.

Removed

• Removed the package build step and Rollup build config for rollup-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 as part of the stack 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]

⚠️ No Changeset found

Latest commit: 04dea10

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[joshblack]

Moved the rollup-plugin-import-css changes into #8044 so they ship with the @primer/react Rolldown migration.