chore: add Sass entry point for @carbon/colors (#19202)

Author: ibacher

packages/colors/package.json

@@ -5,6 +5,7 @@
   "license": "Apache-2.0",
   "main": "lib/index.js",
   "module": "es/index.js",
+  "sass": "index.scss",
   "repository": {
     "type": "git",
     "url": "https://github.com/carbon-design-system/carbon.git",