fix(plugin-preview): do not ignore node_modules in watchOptions

[Timeless0911]

Summary

The preview components is located in node_modules/.rspress/virtual-demo/.

We should watch node_modules to enable hot update in preview block.

Related Issue

fix: #1652

web-infra-dev/rsbuild#4058

Checklist

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

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