Expose shutdownInProgress to the API. (#1485)

Co-authored-by: kyngs <kyngs@users.noreply.github.com>

api/src/main/java/com/velocitypowered/api/proxy/ProxyServer.java

@@ -41,6 +41,13 @@ public interface ProxyServer extends Audience {
    */
   void shutdown();
 
+  /**
+   * Returns whether the proxy is currently shutting down.
+   *
+   * @return {@code true} if the proxy is shutting down, {@code false} otherwise
+   */
+  boolean isShuttingDown();
+
   /**
    * Closes all listening endpoints for this server.
    * This includes the main minecraft listener and query channel.