Support for Vue 2

[crutchcorn]

It would be awesome to support Vue 2 through the @vue/reactivity package. This would need:

• Updates to the build system to publish both Vue 3 and 2 packages
• Updates to our tests to validate it works for both Vue 2 and 3
• Any required updates to the code itself

[crutchcorn]

This work will not make it into v1 due to testing challenges. Open to PRs.

WIP branch is vue-2