feat: A shutting down state in ShutdownHandler (#1454)

### Motivation
Before this PR, there was no way of figuring if cloudnet is shutting
down or not outside of internal cloudnet packages (without using
reflections), causing custom load balancing modules unable to determine
whether it should start new services.

### Modification
This PR exposes a proper function to determine whether a shutdown is in
progress or not.

### Result
Custom modules can now determine whether cloudnet is shutting down or
not

node/src/main/java/eu/cloudnetservice/node/TickLoop.java

@@ -124,6 +124,10 @@ public TickLoop(
     return task;
   }
 
+  public boolean running() {
+    return RUNNING.get();
+  }
+
   public void pause() {
     this.tickPauseRequests.incrementAndGet();
   }