fix(esm): ensure index.mjs is compatible with rollup umd build

Author: remarkablemark

esm/index.mjs

@@ -1,3 +1,4 @@
 import StyleToObject from '../cjs/index.js';
 
-export default StyleToObject.default;
+// ensure compatibility with rollup umd build
+export default StyleToObject.default || StyleToObject;