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

**Use case**
People are used to Postgres.
{
    ASTPtr node_comp;
    if (!elem_parser.parse(pos, node_comp, expected))
        return false;
    ParserKeyword s_is{"IS"};
    ParserKeyword s_not{"NOT"};
    ParserKeyword s_null{"NULL"};
    if (s_is.ignore(pos, expected))
    {
        bool is_not = false;
        if (s_not.ignore(pos, expected))
            is_not = true;
        if (!s_null.ignore(pos, expected))
            return false;
        auto args = std::make_shared<ASTExpressionList>();
        args->children.push_back(node_comp);
        auto function = std::make_shared<ASTFunction>();
        function->name = is_not ? "isNotNull" : "isNull";
        function->arguments = args;
        function->children.push_back(function->arguments);
        node = function;
    }
    else
        node = node_comp;
    return true;
}
bool ParserDateOperatorExpression::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)