fix: Add types file to package.json files (#297)

Author: benjaminparnell

package.json

@@ -4,7 +4,8 @@
   "description": "Seamless integration between Rollup and PostCSS",
   "main": "dist/index.js",
   "files": [
-    "dist"
+    "dist",
+    "types/index.d.ts"
   ],
   "scripts": {
     "test": "npm run lint && jest",