fix(nuxt)!: use parser to generate page metadata

[danielroe]

🔗 Linked issue

resolves nuxt/nuxt#15115, resolves nuxt/framework#8059, resolves nuxt/nuxt#12919, resolves nuxt/nuxt#14483, resolves nuxt/nuxt#15282, resolves nuxt/nuxt#14650, resolves nuxt/nuxt#14742

related nuxt/nuxt#14584

❓ 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 reworks the implementation of definePageMeta to use the built-in parser, and complete overwrite the source file. The previous implementation was causing vite + webpack to mark any imports into the page as dependencies of the metadata, which meant that (in development) vite initial load was sigificantly affected negatively. And in production, both bundlers were rendering far too many prefetches into the HTML.

In comparison, in just our test fixture, this PR reduces:

• network requests from 40 -> 30
• total transferred from 224kB -> 179kB
• total resources from 187kB -> 167kB

👉 Breaking changes

As we are parsing now, a more expensive operation, we now restrict transforms. Only pages within a pages directory (or a layer's pages directory) are scanned for page metadata. If a user is adding a page through a hook (which is the other way for pages to be registered), they can add the metadata manually at that point, rather than relying on definePageMeta extraction.

📝 Checklist

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

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