add method to get first step from flow

Author: Michael Zapf

spec/Model/MultiStepFlowSpec.php

@@ -123,4 +123,14 @@ function it_can_not_get_step_after_last(
 
         $this->getStepAfter($stepCurrent)->shouldReturn(null);
     }
+
+    function it_can_get_first_step(
+        MultiStepInterface $firstStep
+    ) {
+        $firstStep->getId()->willReturn('first-step');
+        $firstStep->getSlug()->willReturn('first-step');
+        $this->addStep($firstStep);
+
+        $this->getFirstStep()->shouldReturn($firstStep);
+    }
 }

src/Model/MultiStepFlow.php

@@ -104,4 +104,13 @@ public function addStep(MultiStepInterface $step): void
         $this->steps[$step->getId()] = $step;
         $this->stepsBySlug[$step->getSlug()] = $step;
     }
+
+    public function getFirstStep(): MultiStepInterface
+    {
+        if (empty($this->steps)) {
+            throw new StepNotFoundException();
+        }
+        reset($this->steps);
+        return current($this->steps);
+    }
 }

src/Model/MultiStepFlowInterface.php

@@ -32,4 +32,6 @@ public function getStepAfter(MultiStepInterface $currentStep): ?MultiStepInterfa
     public function getStepBefore(MultiStepInterface $currentStep): ?MultiStepInterface;
 
     public function addStep(MultiStepInterface $step): void;
+
+    public function getFirstStep(): MultiStepInterface;
 }