Expose MinecraftServer#isRunning

This allows for plugins to detect if the server is actually turning off in onDisable rather than just plugins reloading.

paper-server/src/main/java/org/bukkit/craftbukkit/CraftServer.java

@@ -2960,5 +2960,10 @@ public final class CraftServer implements Server {
     public int getCurrentTick() {
         return net.minecraft.server.MinecraftServer.currentTick;
     }
+
+    @Override
+    public boolean isStopping() {
+        return net.minecraft.server.MinecraftServer.getServer().hasStopped();
+    }
     // Paper end
 }