default FunctionalCommandBuilder<T> assertPermission(String permission) {
        return assertPermission(permission, null);
    }
Asserts that the sender has the specified permission, and sends them the default failure message
if they don't have permission.

@param permission the permission to check for
@return the builder instance
helper/src/main/java/me/lucko/helper/command/functional/FunctionalCommandBuilder.java