Drop support for preact

[emmatown]

What:
Remove all the preact packages and stuff in the build setup for it.

Why:
Supporting preact is getting impractical, it was easy in the past when there were very few differences in the react and preact APIs but as react has added new APIs like createContext, forwardRef and Fragment, it's getting significantly harder. While there are implementations of the new context API for preact, they aren't as performant as React's createContext and they can't have some of the upcoming features like unstable_read and static contextType = MyContext.(before anyone says preact is implementing x and y, this isn't just about the current APIs that are missing in preact, it's about future APIs that we couldn't fully utilize because we would still have to support preact) Also, preact-emotion has significantly fewer downloads (~1k vs ~150k downloads a week) than react-emotion so the low demand for it doesn't justify the cost of maintaining it.

Note that the emotion package will still be usable with preact and if someone wants to maintain a separate implementation of styled for preact, I'd be happy to give up the preact-emotion name on npm.

How:

Checklist:

• Documentation N/A
• Tests N/A
• Code complete

[codecov]

Codecov Report

Merging #896 into master will increase coverage by 0.34%.
The diff coverage is 75%.

Impacted Files	Coverage Δ
packages/core/src/context.js	100% <100%> (ø)	⬆️
packages/styled-base/src/index.js	98.66% <66.66%> (-0.06%)	⬇️
...-plugin-emotion/src/utils/get-target-class-name.js	100% <0%> (ø)	⬆️

[Ailrun]

emotion/packages/is-prop-valid/src/props.js

Lines 463 to 466 in 7ea1715

	// preact
	for: true,
	class: true,
	autofocus: true

is also for Preact, isn't it?

[Andarist]

@Ailrun right, but at the moment is-prop-valid package is also used by other libraries (i.e. styled-components) and they expect preact support, considering that those are just 3 props, I would be in favour of keeping them here

[emmatown]

I agree with @Andarist.