feat(eslint-config): update @antfu/eslint-config to v8

- Bump @antfu/eslint-config from ^7.7.0 to ^8.0.0
- Fix package.json exports to match tsdown output (.mjs/.d.mts)
- Fix README markdown code block lint errors

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: amondnet