fix(nuxt): don't debounce watcher, and include layers

[danielroe]

🔗 Linked issue

resolves nuxt/nuxt#13791, resolves nuxt/nuxt#14053

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 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)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

In some circumstances the watcher was swallowing 'add' events, causing nuxt/nuxt#13791 (not just on Windows), because it was debouncing an add + change event into a single change event. This PR removes the debouncing on the watcher, as we already have a debouncer on the generateApp function. (But alternative suggestions welcome.)

This PR also adds layers to the watch sources, and slightly tightens what paths trigger a regeneration of the app. Layers will be excluded automatically if they are in node_modules, of course, so the effect is only to include base layers within a monorepo.

📝 Checklist

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

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[atinux]

Looking good! It is also a great DX improvement for theme authors.