[9.x] Add Conditionable support for callback conditions

[JosephSilber]

This allows passing a Closure as the conditional to when and unless:

return $this->getOptions()
    ->unless(fn ($options) => $options->has('share'))
    ->put('share', true);

Or even using a higher order collection proxy:

return $this->getOptions()
    ->unless->has('share')
    ->put('share', true);

❤️

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Here's a real-world example, converting @jbrooksuk's Forge code snippet from this:

$jobData = $payload['data'];

if (! isset($jobData['initiated_by'])) {
    $jobData = array_merge($payload['data'], array_filter([
        'initiated_by' => request()->user()->id ?? null,
    ]));
}

return ['data' => $jobData];

...to a collection pipeline:

return collect($payload['data'])
    ->unless->has('initiated_by')
    ->put('initiated_by', request()->user()->id ?? null)
    ->filter()
    ->pipe(fn ($data) => ['data' => $data->all()]);

😄

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Note: this includes a little breaking change. Previously, passing a closure would always be considered truthy, and wouldn't be executed.