`INTERVAL '1 hour'` should be equivalent to `INTERVAL 1 HOUR`

**Use case**
People are used to Postgres.
{
    auto begin = pos;
    
    if (!ParserKeyword("TIMESTAMP").ignore(pos, expected))
        return next_parser.parse(pos, node, expected);
    ASTPtr expr;
    if (!ParserStringLiteral().parse(pos, expr, expected))
    {
        pos = begin;
        return next_parser.parse(pos, node, expected);
    }
    
    auto function = std::make_shared<ASTFunction>();
    
    auto exp_list = std::make_shared<ASTExpressionList>();
    
    function->name = "toDateTime";
    function->arguments = exp_list;
    function->children.push_back(exp_list);
    exp_list->children.push_back(expr);
    node = function;
    return true;
}
bool ParserIntervalOperatorExpression::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)