#276: Add accessors to Wither's invulnerability ticks

src/main/java/org/bukkit/entity/Wither.java

@@ -39,6 +39,22 @@ public interface Wither extends Monster, Boss {
     @Nullable
     LivingEntity getTarget(@NotNull Head head);
 
+    /**
+     * Returns the wither's current invulnerability ticks.
+     *
+     * @return amount of invulnerability ticks
+     */
+    int getInvulnerabilityTicks();
+
+    /**
+     * Sets the wither's current invulnerability ticks.
+     *
+     * When invulnerability ticks reach 0, the wither will trigger an explosion.
+     *
+     * @param ticks amount of invulnerability ticks
+     */
+    void setInvulnerabilityTicks(int ticks);
+
     /**
      * Represents one of the Wither's heads.
      */