Add ability to check for command alias existence in api module

api/src/main/java/com/velocitypowered/api/command/CommandManager.java

@@ -122,4 +122,12 @@ public interface CommandManager {
    *         Can be completed exceptionally if an exception is thrown during execution.
    */
   CompletableFuture<Boolean> executeImmediatelyAsync(CommandSource source, String cmdLine);
+
+  /**
+   * Returns whether the given alias is registered on this manager.
+   *
+   * @param alias the command alias to check
+   * @return {@code true} if the alias is registered
+   */
+  boolean hasCommand(String alias);
 }

proxy/src/main/java/com/velocitypowered/proxy/command/VelocityCommandManager.java

@@ -227,6 +227,7 @@ public class VelocityCommandManager implements CommandManager {
    * @param alias the command alias to check
    * @return {@code true} if the alias is registered
    */
+  @Override
   public boolean hasCommand(final String alias) {
     Preconditions.checkNotNull(alias, "alias");
     return dispatcher.getRoot().getChild(alias.toLowerCase(Locale.ENGLISH)) != null;