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

**Use case**
People are used to Postgres.
bool ParserKeyValuePair::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
{
    ParserIdentifier id_parser;
    ParserLiteral literal_parser;
    ASTPtr identifier;
    ASTPtr value;
    bool with_brackets = false;
    if (!id_parser.parse(pos, identifier, expected))
        return false;
    
    if (!literal_parser.parse(pos, value, expected) && !id_parser.parse(pos, value, expected))
    {
        ParserKeyValuePairsList kv_pairs_list;
        ParserToken open(TokenType::OpeningRoundBracket);
        ParserToken close(TokenType::ClosingRoundBracket);
        if (!open.ignore(pos))
            return false;
        if (!kv_pairs_list.parse(pos, value, expected))
            return false;
        if (!close.ignore(pos))
            return false;
        with_brackets = true;
    }
    auto pair = std::make_shared<ASTPair>(with_brackets);
    pair->first = Poco::toLower(typeid_cast<ASTIdentifier &>(*identifier.get()).name);
    pair->set(pair->second, value);
    node = pair;
    return true;
}
bool ParserKeyValuePairsList::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)