Preact toChildArray does not assign keys

[just-boris]

Reproduction

https://fanatical-dot-bumper.glitch.me/

In React documentation, they say

That is, toArray prefixes each key in the returned array so that each element’s key is scoped to the input array containing it.

In other words, it will assign keys, even if they were undefined before. This is not the case for Preact's implementation and causes issues when using preact/compat

[marvinhagemeister]

So I've went a bit back and forth and although I made a PR, I'm struggling to find a use case for this. @just-boris can you elaborate on the scenario you're needing this change in?

[just-boris]

I have code which uses react-keyed-flatten-children and it produces warnings after passing through preact/compact's implementation.