perf(nuxt): improve link prefetching

[danielroe]

🔗 Linked issue

❓ 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

This PR bundles a number of improvements to the link prefetching feature of <NuxtLink>:

1. It implements crossorigin prefetches using the Speculation Rules API - this is behind a flag for experimentation.
2. It disables prefetching for links with target: _blank as there is no benefit to doing so.
3. It throttles preload requests (as, the way we do it, these are high priority requests and could block other more urgent user interaction with the page). This implementation could likely be improved but I aimed to keep it lightweight.

📝 Checklist

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

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