Auto enable SSR based on existence of SSR bundle

[reinink]

This PR removes the inertia.ssr.enabled config value in favor of automatically enabling SSR based on the existence of an SSR JavaScript bundle file.

By default it looks for the SSR bundle in a few common places, so quite often you won't even need to publish an Inertia config as long as you're using a pretty standard setup, such as bootstrap/ssr/ssr.mjs — the default location used by the Laravel Vite plugin.

You can, of course, still set a custom SSR bundle path using the inertia.ssr.bundle config option:

'ssr' => [

    'url' => 'http://127.0.0.1:13714',

    'bundle' => base_path('bootstrap/ssr/ssr.mjs'), // <!-- here

],

This PR also updates the inertia:start-ssr command to automatically terminate the node process should the PHP process die, as well as when the command is run, just in case there is a lingering node process hanging around still.