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

[danielroe]

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.