Use sealed types in command interfaces (#1239)

* Use sealed types in command interfaces

* Some minor code fixes

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

@@ -29,5 +29,5 @@ import com.velocitypowered.api.proxy.Player;
  *
  * </ul>
  */
-public interface Command {
+public sealed interface Command permits BrigadierCommand, InvocableCommand {
 }

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

@@ -21,7 +21,8 @@ import java.util.concurrent.CompletableFuture;
  *
  * @param <I> the type of the command invocation object
  */
-public interface InvocableCommand<I extends CommandInvocation<?>> extends Command {
+public sealed interface InvocableCommand<I extends CommandInvocation<?>> extends Command
+        permits RawCommand, SimpleCommand {
 
   /**
    * Executes the command for the specified invocation.

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

@@ -12,7 +12,7 @@ package com.velocitypowered.api.command;
  * the command and its arguments directly without further processing.
  * This is useful for bolting on external command frameworks to Velocity.
  */
-public interface RawCommand extends InvocableCommand<RawCommand.Invocation> {
+public non-sealed interface RawCommand extends InvocableCommand<RawCommand.Invocation> {
 
   /**
    * Contains the invocation data for a raw command.

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

@@ -16,7 +16,7 @@ import org.checkerframework.checker.nullness.qual.NonNull;
  * <p>Prefer using {@link BrigadierCommand}, which is also
  * backwards-compatible with older clients.
  */
-public interface SimpleCommand extends InvocableCommand<SimpleCommand.Invocation> {
+public non-sealed interface SimpleCommand extends InvocableCommand<SimpleCommand.Invocation> {
 
   /**
    * Contains the invocation data for a simple command.