Correct an issue with string stepDefinitions

This fixes #781.

features/issues/781.feature

@@ -0,0 +1,32 @@
+# https://github.com/badeball/cypress-cucumber-preprocessor/issues/781
+
+Feature: non-array step definitions, not matching
+  Scenario: non-array step definitions, not matching
+    Given additional preprocessor configuration
+      """
+      {
+        "stepDefinitions": "foo/bar"
+      }
+      """
+    And a file named "cypress/e2e/a.feature" with:
+      """
+      Feature: a feature
+        Scenario: a scenario
+          Given an undefined step
+      """
+    When I run cypress
+    Then it fails
+    And the output should contain
+      """
+      Step implementation missing for "an undefined step".
+
+      We tried searching for files containing step definitions using the following search pattern templates:
+
+        - foo/bar
+
+      These templates resolved to the following search patterns:
+
+        - foo/bar
+
+      These patterns matched **no files** containing step definitions. This almost certainly means that you have misconfigured `stepDefinitions`.
+      """

lib/create-tests.ts

@@ -53,7 +53,7 @@ interface CompositionContext {
     stack: messages.IEnvelope[];
   };
   stepDefinitionHints: {
-    stepDefinitions: string[];
+    stepDefinitions: string | string[];
     stepDefinitionPatterns: string[];
     stepDefinitionPaths: string[];
   };
@@ -862,7 +862,7 @@ function createMissingStepDefinitionMessage(
     .replaceAll("<text>", text)
     .replaceAll(
       "<step-definitions>",
-      prettyPrintList(stepDefinitionHints.stepDefinitions)
+      prettyPrintList([stepDefinitionHints.stepDefinitions].flat())
     )
     .replaceAll(
       "<step-definition-patterns>",