fix: augment vue rather than @vue/runtime-core

[danielroe]

🔗 Linked issue

vuejs/router#2295

❓ 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)
• ✨ 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

In line with vuejs/router#2295 and nuxt/nuxt#28542, this moves to augment vue rather than @vue/runtime core.

This is now officially recommended in the docs and it must be done by all libraries or it will break types for other libraries.