Add isHardcore API to check if the server is in hardcore mode or not. Adds BUKKIT-2707

By: EvilSeph <evilseph@gmail.com>

paper-api/src/main/java/org/bukkit/Bukkit.java

@@ -243,6 +243,10 @@ public final class Bukkit {
         return server.getAllowFlight();
     }
 
+    public static boolean isHardcore() {
+        return server.isHardcore();
+    }
+
     public static void shutdown() {
         server.shutdown();
     }