chore: add private: true in package.json (sveltejs#225)

This settings prevents users from accidentally publishing their svelte code to an NPM registry. Almost no user will want to publish their app to NPM, and if they wish to do so they can safely set private to false.

package.json

@@ -1,6 +1,7 @@
 {
   "name": "svelte-app",
   "version": "1.0.0",
+  "private": true,
   "scripts": {
     "build": "rollup -c",
     "dev": "rollup -c -w",