Extract "css" variable in JS for custom Server Side Rendering

Hi,

We use css-modules in a react app. In our webpack based web app, in order to collect styles for the critical components, we basically wrap those components into a HOC. This decorator basically accumulates all the generated CSS and inlines it into the HTML while server side rendering.

Example;

```jsx
// component
import styles from "./styles.scss";
import { withStyles } from "utils/with-styles";

function Component (props) {
return

;
}

export default withStyles(Component, styles);
```

```jsx
// utils/with-styles.js
export function withStyles(Component, styles) {
const context = useContext(CriticalCSSContext);

if(context && context.addStyles) {
if (!context.criticalStyles.has(styles._getId) {
context.addStyles(styles._getId(), styles._getCss());
}
}

return (props) => ;
}
```

```jsx
// index.js
function App() {
const criticalStyles = useRef({});
const criticalCss = {
addStyles: (id, css) => {
criticalStyles[id].current = css
},
criticalStyles: {
has: (id) => criticalStyles.current[id],
},
};
return (



);
}
```

While we can access the className-generatedClassName map, we can't quite get the actual "css" content. To access the generated CSS, we wrote a custom style loader that exports the CSS content.

I was wondering if the same could be achieved by getting a onExport hook/plugin/loader where we can essentially add `module.exports.css = css;` line in the final css module file.

This is the style loader we wrote for webpack;

```js
// custom_webpack_style_loader.js
var stringifyRequest = require("loader-utils").stringifyRequest;

module.exports = function loader() {};

module.exports.pitch = function pitch(remainingRequest) {
if (this.cacheable) {
this.cacheable();
}

return `
var content = require(${stringifyRequest(this, `!!${remainingRequest}`)});
if (typeof content === 'string') {
content = [[module.id, content, '']];
}
module.exports = content.locals || {};
module.exports._getContent = () => content;
module.exports._getCss = () => content.toString();
module.exports._getId = () => module.id;
`;
};
```

I'm sorry if this is beyond the scope of this plugin, but would love some pointers to support this. Thank you so much for this wonderful plugin!