fix(nuxt): improve warning for invalid children of <Title>

[NozomuIkuta]

🔗 Linked issue

Resolves #19716

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

I improved a logic to detect wrong usage of Title component.

I tested the following usages in playground:

<Title>Valid</Title>
<Title>{{ 'valid' }}</Title>
<Title>{{ 'valid' }} {{ 'valid' }}</Title>
<Title><p>valid</p></Title>

<!-- Error -->
<Title>
  error due to more than
  <p>2 nodes</p>
</Title>
<Title>
  <!-- Comment is also a vnode -->
  so this is 2nd node
</Title>

We could use VNode's type property to detect redundant p tag and/or preceding comment tag,
but I think it's not a good idea to rely on undocumented APIs.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

I couldn't find the corresponding part of documentation.
If you let me know, I will modify it too.

[danielroe]

Thank you! ❤️