Install Broadcasting Command Fix for Livewire Starter Kit

[joshcirre]

When playing around with the new useEcho hook for the starter kits, I realized that the Broadcasting Install command no longer worked seamlessly for the Livewire starter kit. I assume this is because there is no more bootstrap.js for this starter kit and no longer needed for most Livewire applications.

That being said, I admittedly spent more time than I should have figuring out I did not have Echo proper instantiated after installing the Broadcasting command, so this PR is set up to add a check and add the echo file import for app.js if and only if bootstrap.js is not found.

This means the install:broadcasting command is the only thing needed to run for any starter kit.