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

[danielroe]

Description

In line with https://github.com/vuejs/router/pull/2295 and https://github.com/nuxt/nuxt/pull/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.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Improvement/refactoring (non-breaking change that doesn't add any feature but make things better)