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

**Use case**
People are used to Postgres.
{
    ParserExpressionWithOptionalAlias elem_p(false);
    ParserKeyword ascending("ASCENDING");
    ParserKeyword descending("DESCENDING");
    ParserKeyword asc("ASC");
    ParserKeyword desc("DESC");
    ParserKeyword nulls("NULLS");
    ParserKeyword first("FIRST");
    ParserKeyword last("LAST");
    ParserKeyword collate("COLLATE");
    ParserKeyword with_fill("WITH FILL");
    ParserKeyword from("FROM");
    ParserKeyword to("TO");
    ParserKeyword step("STEP");
    ParserStringLiteral collate_locale_parser;
    ParserExpressionWithOptionalAlias exp_parser(false);
    ASTPtr expr_elem;
    if (!elem_p.parse(pos, expr_elem, expected))
        return false;
    int direction = 1;
    if (descending.ignore(pos) || desc.ignore(pos))
        direction = -1;
    else
        ascending.ignore(pos) || asc.ignore(pos);
    int nulls_direction = direction;
    bool nulls_direction_was_explicitly_specified = false;
    if (nulls.ignore(pos))
    {