Checks if an argument contains variadic characters.

@example [arg...] >> true

@param val the value to be inspected.
@param variadic the variadic char to check if exists.
function isArgVariadic(val, variadic) {
    variadic = variadic || constants_1.VARIADIC_CHAR;
    return isArg(val)
        && (val.endsWith(variadic + ']')
            || val.endsWith(variadic + '>'));
}