feat(server): support viewing served files via /rsbuild-dev-server route

[hangCode2001]

Summary

visit /rsbuild-dev-server route will show where files are served. For example,
http://localhost:3001/rsbuild-dev-server.

Related Links

#3519

Checklist

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

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[CLAassistant]

All committers have signed the CLA.

[9aoy]

need signed CLA~

[chenjiahan]

Thanks!