Move laravel-mix to production dependencies

package.json

@@ -3,6 +3,7 @@
   "private": true,
   "dependencies": {
     "@rails/ujs": "^6.0.0",
+    "laravel-mix": "^6.0.28",
     "normalize-scss": "^7.0.1",
     "tufte-css": "^1.8.0",
     "turbolinks": "^5.2.0"
@@ -17,7 +18,6 @@
     "@babel/compat-data": "^7.9.0",
     "browser-sync": "^2.26.7",
     "compression-webpack-plugin": "^7.0.0",
-    "laravel-mix": "^6.0.0",
     "postcss": "^8.1.0",
     "resolve-url-loader": "^3.1.1",
     "sass": "^1.26.10",