feat: add difficulty setting

src/main/java/eu/midnightdust/yaytris/util/Difficulty.java

@@ -0,0 +1,14 @@
+package eu.midnightdust.yaytris.util;
+
+public enum Difficulty {
+    NOOB(2000), EASY(1200), NORMAL(1000), HARD(750), EXTREME(500), WTF(100);
+    private final int timerPeriod;
+
+    Difficulty(int timerPeriod) {
+        this.timerPeriod = timerPeriod;
+    }
+
+    public int getTimerPeriod() {
+        return timerPeriod;
+    }
+}