Prebuilt extensions should use styleModule

[jasongrout]

In #9427, we introduced a new styleModule key instead of guessing about there being a js file corresponding to a style file. That change did not propagate to

jupyterlab/builder/src/extensionConfig.ts

Lines 67 to 77 in 6ff9516

	if (data.style) {
	let style = path.join(packagePath, data.style);
	if (path.extname(style) === '.css') {
	// See if there is a corresponding js file we can load instead
	const jsFile = `${style.slice(0, style.length - 4)}.js`;
	if (fs.existsSync(jsFile)) {
	style = jsFile;
	}
	}
	exposes['./style'] = style;
	}

The above code should be updated to use the styleModule key.