PHP 8.4 support

[Sweetchuck]

Current code (v0.2.2) throws deprecation messages:

Deprecated: Graphp\GraphViz\Dot::__construct(): Implicitly marking parameter $graphviz as nullable is deprecated, the explicit nullable type must be used instead in [/.../src/Dot.php] on line 13

[PaulRotmann]

Thanks for reporting this PHP 8.4 compatibility issue. You're correct, we haven't added PHP 8.4 support to our testing yet.
The nullable parameter deprecation should be straightforward to fix.

Contributions are welcome if you'd like to submit a PR, otherwise we'll address this when we have resources available.