Add support for Laravel Scout 9 and Laravel 9

[ikari7789]

Adds

• GitHub Actions for running tests against supported versions.
• Support for PHP 8.
• Support for Laravel 9.

Changes

• Minimum supported PHP version to ^7.3.
• Laravel Scout version to ^9.0.
• Supported wamania/php-stemmer version to v3 and allows passing in the stemmer object directly.

[michaelbaril]

Hey @ikari7789 , that's great, thanks a lot for your PR! And sorry for merging it so ridiculously late...