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

**Use case**
People are used to Postgres.
{
    ParserNull null_p;
    ParserNumber num_p;
    ParserStringLiteral str_p;
    if (null_p.parse(pos, node, expected))
        return true;
    if (num_p.parse(pos, node, expected))
        return true;
    if (str_p.parse(pos, node, expected))
        return true;
    return false;
}
const char * ParserAlias::restricted_keywords[] =
{
    "FROM",
    "FINAL",
    "SAMPLE",
    "ARRAY",
    "LEFT",
    "RIGHT",
    "INNER",
    "FULL",