fix: lazy import webpack-dev-middleware

[chenjiahan]

Summary

After merging #2361, an unexpected rebuild will occur when Rspress starts:

Then I found that lazy import webpack-dev-middleware can solve this issue, this may be some side effects of webpack-dev-middleware on fs.

This PR lazy import webpack-dev-middleware to fix this issue.

Checklist

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

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