Add SimpleCommand.Invocation#alias function.

It seems like this ability is generally useful outside RawCommand, so let's add this to SimpleCommand too.

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

@@ -16,5 +16,11 @@ public interface SimpleCommand extends InvocableCommand<SimpleCommand.Invocation
    */
   interface Invocation extends CommandInvocation<String @NonNull []> {
 
+    /**
+     * Returns the used alias to execute the command.
+     *
+     * @return the used command alias
+     */
+    String alias();
   }
 }

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

@@ -15,11 +15,21 @@ final class VelocitySimpleCommandInvocation extends AbstractCommandInvocation<St
     @Override
     public SimpleCommand.Invocation create(final CommandContext<CommandSource> context) {
       final String[] arguments = BrigadierUtils.getSplitArguments(context);
-      return new VelocitySimpleCommandInvocation(context.getSource(), arguments);
+      final String alias = BrigadierUtils.getAlias(context);
+      return new VelocitySimpleCommandInvocation(context.getSource(), alias, arguments);
     }
   }
 
-  VelocitySimpleCommandInvocation(final CommandSource source, final String[] arguments) {
+  private final String alias;
+
+  VelocitySimpleCommandInvocation(final CommandSource source, final String alias,
+      final String[] arguments) {
     super(source, arguments);
+    this.alias = alias;
+  }
+
+  @Override
+  public String alias() {
+    return this.alias;
   }
 }