Fix named exports, add css-loader 7 support

[jdelStrother]

css-loader 7 defaults to named exports in the css - eg

// Imports
import ___CSS_LOADER_API_NO_SOURCEMAP_IMPORT___ from "../node_modules/css-loader/dist/runtime/noSourceMaps.js";
import ___CSS_LOADER_API_IMPORT___ from "../node_modules/css-loader/dist/runtime/api.js";
import ___CSS_LOADER_ICSS_IMPORT_0___, * as ___CSS_LOADER_ICSS_IMPORT_0____NAMED___ from "-!../node_modules/css-loader/dist/cjs.js??ruleSet[1].rules[0].use[1]!./example-shared.css";
var ___CSS_LOADER_EXPORT___ = ___CSS_LOADER_API_IMPORT___(___CSS_LOADER_API_NO_SOURCEMAP_IMPORT___);
___CSS_LOADER_EXPORT___.i(___CSS_LOADER_ICSS_IMPORT_0___, "", true);
// Module
___CSS_LOADER_EXPORT___.push([module.id, ".nebQsrLtrM33XOPOAV0d {\n  color: white;\n}\n\n.vUAoNtkdxlUMvNCQvVEj {\n  color: green;\n}\n\n.Z8l2Dbal4vZ3rn2gu33u {\n}\n", ""]);
// Exports
export var foo = "nebQsrLtrM33XOPOAV0d";
var _1 = "vUAoNtkdxlUMvNCQvVEj";
export { _1 as "bar-baz" };
export var composed = "Z8l2Dbal4vZ3rn2gu33u " + ___CSS_LOADER_ICSS_IMPORT_0____NAMED___["shared-style"] + "";
export default ___CSS_LOADER_EXPORT___;

I've added css-loader 7 to the test suite and fixed getCssModuleKeys to support both styles of exports.

Fixes #65

[jdelStrother]

Anything we can do to help get this merged?