feat(core): add html language attribute

[lucioreyli]

Summary

Related Links

https://dequeuniversity.com/rules/axe/4.10/html-has-lang
https://www.iana.org/assignments/language-subtag-registry/language-subtag-registry

Required by PageSpeed Insights

Checklist

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

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Lighthouse	1 paths audited Performance: 56 (🟢 up 7 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

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[lucioreyli]

The proposal of this feature is to not create a extra html file, just editing the default entry html file and fixing this accessibility issue

[CLAassistant]

All committers have signed the CLA.

[chenjiahan]

Thank you for your contribution!

My concern is that adding a default setting of lang="en" might not be friendly for non-English language sites.

Another concern is whether we need to add a configuration for lang, as Rsbuild cannot add configuration for all common HTML attributes. Perhaps it is a better way to use html.template directly to fully control the template.