Add has step checks

[Bird87ZA]

I've added functionality to be able to see if there are succeeding or preceding steps in the wizard.

This is useful because it allows you to have the following:

@if ($this->hasPreviousStep())
    <button wire:click="previousStep">
        {{ __('Back') }}
    </button>
@endif

@if ($this->hasNextStep())
    <button wire:click="nextStep">
        {{ __('Next') }}
    </button>
@else
    <button wire:click="submit">
        {{ __('Create') }}
    </button>
@endif

This is a new PR related to #76

[freekmurze]

Looks good, only a tiny remark on the tests

[freekmurze]

Thank you!