fix(webpack): use html-webpack-plugin by default

[chenjiahan]

Summary

Use html-webpack-plugin in @rsbuild/webpack by default.

This allows us to:

• Remove __internalHelper from @rsbuild/core. We no longer need to export the helper for Modern.js
• @rsbuild/core can switch to the Rust HtmlRspackPlugin in the future.

Checklist

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

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