types: export HtmlRspackPlugin type

[chenjiahan]

Summary

Export HtmlRspackPlugin type for plugins to use.

This helps plugin-rem or plugin-assets-retry to remove the html-rspack-plugin dependency. We should provide more HTML-related plugin hooks for Rsbuild plugins in the future to reduce the dependency on html-rspack-plugin hooks.

Checklist

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

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