Smooth migration to the Vue 3 with @vue/composition-api [proposal]

[shapkarin]

I suggest to find a time in the future to add @vue/composition-api that can help to use some Vue 3 features with the the Vue 2. Next it can help to slightly migrate all project to the Vue 3.

https://github.com/vuejs/composition-api
https://composition-api.vuejs.org

[davidyuk]

As I know vue3 supports old component syntax, so it shouldn't be the first step to migrate to vue 3. I would wait for the release of @vue/cli@5. Somebody needs to check that dependencies are not compatible with vue 3.

this issue related to #1047