Remove all traces of Preact from the repository

[srmagura]

This was originally discussed here.

This PR is a stepping stone for the Yarn 3 upgrade. This is necessary because we depended on a really old version of Preact that Yarn 3 failed to install. (Yarn 3 upgrade is blocked by the website upgrade, btw. That is #2571.)

If anyone is using @emotion/styled with Preact, my changes to isPropValid will break them. But I kind of doubt @emotion/styled works with Preact in the first place so it should be fine. (?)

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[Andarist]

It looks like those were actually working :p although I guess that there is no special handling for this in our code?

[srmagura]

Lol, so yes the snapshots were working, though we were on a very old version of Preact and using preact-render-to-json which is now defunct AFAIK.

I guess, bottom line is, I don't see the need for Preact tests if Emotion doesn't support Preact.