SX: change imports/exports to allow composability

[mrtnzlml]

I would like to propose a change of the default export which would allow us to merge external SX stylesheets. For example, here is a base component:

import sx from '@adeira/sx';

type Props = {|
  +xstyle: {|
    +marginTop: number,
  |},
|};

const styles = sx.create({ default: { fontSize: 16 } });

function MyBaseComponent(props: Props) {
  return <div className={sx(styles.default, props.xstyle)} />;
}

And another wrapping component which is sending down some custom styles:

const styles = sx.create({ spacing: { marginTop: 4 } });

function MyCustomComponent() {
  return <MyBaseComponent xstyle={styles.spacing} />;
}

We are currently unable to achieve such behavior without this change.

This change is inspired by StyleX, see: https://youtu.be/9JZHodNR184

[tbergquist-godaddy]

LGTM