Add official scoped preact preset, convert to latest Neutrino API

[eliperelman]

This PR merges in the preact preset to packages, with a little cleanup to match the latest Neutrino API. This also adds the automatic import of h so users don't need to do it:

// not needed, this happens automatically
import { h } from 'preact';

In order to make the react preset consistent, I've also implemented the same auto-import of createElement from react:

// not needed, happens automatically
import React from 'react';

This would fix #260.

[edmorley]

I see a few small differences between this imported preset and the existing react one - I've filed #473 to sync them up at some point.