[Feature request] Add Support for Laravel 9

[arminlinzbauer]

Laravel 9 is around the corner (due to launch early next year), so some people like me are already actively developing on the new Laravel 9 platform, whether to push out their updates to Laravel 9 as soon as possible or to avoid having to update a new project after only a few weeks when Laravel 9 finally comes around.

It would be great if the laravel-debugbar package supported it already.

[arminlinzbauer]

I also submitted a PR to php-debugbar (php-debugbar/php-debugbar#492) to resolve the issue with Laravel 9's Symfony dependencies, so composer install will only work if this PR is accepted as well.