feat(plugin-less): update webpackImporter type to support 'only'

[chenjiahan]

Summary

Updated the webpackImporter property to support a new only option, allowing exclusive use of the built-in Rspack resolver. Added detailed documentation to clarify its behavior and default value.

Related Links

• feat: Add support for using only webpackImporter webpack-contrib/less-loader#562

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[netlify]

✅ Deploy Preview for rsbuild ready!

Name	Link
🔨 Latest commit	7c9fa32
🔍 Latest deploy log	https://app.netlify.com/sites/rsbuild/deploys/681ad70de0bf620008b0394d
😎 Deploy Preview	https://deploy-preview-5181--rsbuild.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
Lighthouse	1 paths audited Performance: 84 (🟢 up 2 from production) Accessibility: 97 (no change from production) Best Practices: 100 (no change from production) SEO: 100 (no change from production) PWA: 60 (no change from production) View the detailed breakdown and full score reports

---

To edit notification comments on pull requests, go to your Netlify site configuration.