Add function exists function

ink-engine-runtime/Story.cs

@@ -963,6 +963,20 @@ public void ChooseChoiceIndex(int choiceIdx)
             ChoosePath (choiceToChoose.choicePoint.choiceTarget.path);
         }
 
+        /// <summary>
+        /// Checks if a function exists.
+        /// </summary>
+        /// <returns>True if the function exists, else false.</returns>
+        /// <param name="functionName">The name of the function as declared in ink.</param>
+        public bool FunctionExists (string functionName)
+        {
+            try {
+                return ContentAtPath (new Path (functionName)) is Runtime.Container;
+            } catch (StoryException e) {
+                return false
+            }
+        }
+
         /// <summary>
         /// Evaluates a function defined in ink.
         /// </summary>