Added Six Meanings test to screen switcher

The screen switcher is now able to switch screens to the Six Meanings test

src/Welsh Vocabulary Tutor/src/main/java/uk/ac/aber/cs22120/group20/javafx/ScreenSwitch.java

@@ -113,6 +113,7 @@ public class ScreenSwitch extends SharedCodeController {
         flashcardScene("flashcard.fxml"),
         practiceListScene("practicelist.fxml"),
         matchMeaningScene("matchthemeaning.fxml"),
+        sixMeaningScene("sixmeanings.fxml"),
         translationScene("translation.fxml"),
         ;