fix: update package.json to include index.css path and remove assets entry

Author: onmax

packages/nimiq-vitepress-theme/package.json

@@ -28,9 +28,7 @@
       "import": "./dist/index.mjs",
       "require": "./dist/index.js"
     },
-    "./assets": {
-      "import": "./dist/assets/*"
-    },
+    "./index.css": "./dist/assets/index.css",  
     "./*": {
       "types": "./dist/*",
       "import": "./dist/*.mjs",