fix: export default postcss plugin in ESM syntax

Author: vnphanquang

.changeset/cold-cougars-grab.md

@@ -0,0 +1,5 @@
+---
+"postcss-color-scheme": patch
+---
+
+export default postcss plugin

lib/postcss.js

@@ -128,4 +128,4 @@ function pluginCreator(_opts = {}) {
  */
 pluginCreator.postcss = true;
 
-module.exports = pluginCreator;
+export default pluginCreator;