[Configuration] Fix parallel config always replaced by next config as default to true

[samsonasik]

@Reinis here to fix #180

The existing ECSConfigBuilder::$parallel property is default to true, so the $ecsConfig->parallel() is always called, instead, it needs to be nullable bool:

private ?bool $parallel = null;

with default to null, and call parallel() or disableParallel() when it enabled/disabled:

        if ($this->parallel !== null) {
            if ($this->parallel) {
                $ecsConfig->parallel(
                    seconds: $this->parallelTimeoutSeconds,
                    maxNumberOfProcess: $this->parallelMaxNumberOfProcess,
                    jobSize: $this->parallelJobSize
                );
            } else {
                $ecsConfig->disableParallel();
            }
        }

To ensure it enabled by default on the project, I set entry point in config/config.php:

# config/config.php
return ECSConfig::configure()
    ->withParallel()

I tried in this repo and it seems working ok 👍

[samsonasik]

All checks have passed 🎉 @TomasVotruba I think it is ready.

[TomasVotruba]

Looks good to me 👍 Thanks