[5.7] Update vue preset's vue-stubs for laravel-mix 4 compatibility

[cthorne91]

The stubs for app.js and ExampleComponent.vue when issuing the artisan preset vue command differ from the laravel project's default app.js and ExampleComponent.vue files. Furthermore, the app.js stub is incompatible with laravel-mix 4 because it does not reference .default.

According to the Laravel-Mix 4 upgrade guide https://laravel-mix.com/docs/4.0/upgrade

As part of the vue-loader 15 updates, if your code uses the CommonJS syntax for importing EcmaScript modules, you'll need to append .default...

How to reproduce
Using a fresh install of laravel, issue the php artisan preset vue command.

I'm thinking this was simply overlooked when upgrading laravel to mix 4, or possibly I'm missing something. Hope this PR helps 👍