Add peer variants

[ben-rogerson]

This PR adds the peer-x variants which can be used like this:

// When the `.peer` element is hovered/focussed, the div recieves `display: block`
<div>
	<button className="peer">Test</button>
	<div tw="peer-hover:peer-focus:block" />
</div>

Here's the conversion that takes place:

import tw from "twin.macro";

tw`peer-hover:peer-focus:block`;

// ↓ ↓ ↓ ↓ ↓ ↓

({
  ".peer:hover:focus ~ &": {
    "display": "block"
  }
});