Add normalize children helper to unify behavior with Vue.js 3

What does this MR do and why?

This Mr adds tiny helper to normalize render function children between Vue.js 2 and Vue.js 3

How to set up and validate locally

VUE_VERSION=3 yarn jest output should not contain any complains about __asyncLoader

Why __asyncLoader?

When Vue.js sees unknown node format (without normalization it will be { default() { ... } it goes through all known node types and when it does not match it assumes that this is async component. Attempt to unmount "async-component" vnode makes Vue.js to complain about __asyncLoader

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