Add throws CommandSyntaxException to BasicCommand

[Strokkur424]

This PR quite literally just adds the throws semantic to the #execute method on BasicCommand's #execute method. That method is only called inside a regular Brigadier .executes block, where CommandSyntaxException can be thrown regularly, meaning no additional try/catch logic is needed.

This change allows you to throw exceptions when implement BasicCommands, including when implementing it with lambdas. This makes the following code valid:

this.registerCommand("test", (_, _) -> {
    throw new SimpleCommandExceptionType(new LiteralMessage("hey")).create();
});