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

[danielroe]

For a while, in the Vue ecosystem we've been augmenting @vue/runtime-core to add custom properties and more to vue. However, this inadvertently breaks the types for projects that augment vue - which is (now?) the officially recommended in the docs way to augment these interfaces (for example, ComponentCustomProperties, GlobalComponents and so on).

This means all libraries must update their code (or it will break the types of the libraries that augment vue instead).

Here's an example of how the augmented types end up broken.

This PR is a small effort to ensure the ecosystem is consistent. For context, you can see that vue-router has moved to do this, as well as Nuxt.

[dlevitas]

Thank you for this PR @danielroe

This looks good, I've merged it into our main repository, much appreciated.