[5.x] Allow custom render callback for overridden exceptions

[FrittenKeeZ]

Upgrading from Statamic 4 and Laravel 10 to Statamic 5 and Laravel 11 I wanted to migrate this custom exception handler:

public function render($request, \Throwable $e)
{
    if ($e instanceof NotFoundHttpException) {
        $eventsMount = Collection::findByHandle('events')->mount();
        if (Str::before($request->path(), '/') === $eventsMount->slug()) {
            return response()->redirectTo($eventsMount->url());
        }
    }

    return parent::render($request, $e);
}

Following Laravel's documentation I couldn't understand why it didn't work, until I dived into the exception and saw it implements a render method which takes over.

This PR introduces a static callback you can use to implement your own handling:

NotFoundHttpException::$renderCallback = function (Request $request) {
    $eventsMount = Collection::findByHandle('events')->mount();
    if (Str::before($request->path(), '/') === $eventsMount->slug()) {
        return response()->redirectTo($eventsMount->url());
    }
};