Custom JSX pragma for better integration with React

## Motivation

Inspired by Emotion, a [`css` prop](https://emotion.sh/docs/css-prop) could be attached to JSX elements for better composition possibilities.

## Basic example

```jsx

I am blue

## Details

For compatibility purposes, the [React Classic Runtime of Babel](https://babeljs.io/docs/en/babel-preset-react#react-classic-runtime) should be supported.

Start by reading the React Classic Runtime of Babel documentation and the linked Emotion css prop example. Define how the custom JSX pragma should handle the shown css prop while remaining compatible with the classic runtime; done means the JSX example works under that compatibility mode.

Written by the indexing model from the issue text.