fix(extras): help vite dep analysis determine type of script quasarframework#12833

Vite reports this:

   @quasar/extras doesn't appear to be written in CJS, but also doesn't appear
   to be a valid ES module (i.e. it doesn't have "type": "module" or an .mjs
   extension for the entry point). Please contact the package author to fix.

Author: rstoenescu

extras/index.js

@@ -1,2 +1,5 @@
-// empty file
+// this file serves no purpose other than
 // to help q/app-webpack discover where this package is installed
+
+// also helping Vite's dep analysis the type of script
+module.exports = ''

extras/package.json

@@ -5,6 +5,7 @@
   "scripts": {
     "build": "node build/index.js"
   },
+  "main": "index.js",
   "repository": {
     "type": "git",
     "url": "git+https://github.com/quasarframework/quasar.git"