fix: npm files

Author: 2nthony

package.json

@@ -4,5 +4,8 @@
   "version": "1.0.0",
   "repository": "https://github.com/evillt/vercel-css-vars.git",
   "author": "evillt <ijoec123@gmail.com>",
-  "license": "MIT"
+  "license": "MIT",
+  "files": [
+    "index.css"
+  ]
 }