Added method to shutdown the server

By: Dinnerbone <dinnerbone@dinnerbone.com>

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

@@ -215,4 +215,8 @@ public final class Bukkit {
     public static boolean getAllowFlight() {
         return server.getAllowFlight();
     }
+
+    public static void shutdown() {
+        server.shutdown();
+    }
 }

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

@@ -346,4 +346,9 @@ public interface Server {
      * @return Whether this server allows flying or not.
      */
     public boolean getAllowFlight();
+
+    /**
+     * Shutdowns the server, stopping everything.
+     */
+    public void shutdown();
 }