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

**Use case**
People are used to Postgres.
#include <Parsers/IAST.h>
#include <Parsers/ASTExpressionList.h>
#include <Parsers/ASTFunction.h>
#include <Parsers/parseIntervalKind.h>
#include <Parsers/ExpressionElementParsers.h>
#include <Parsers/ExpressionListParsers.h>
#include <Parsers/ParserCreateQuery.h>
#include <Parsers/ASTFunctionWithKeyValueArguments.h>
#include <Common/typeid_cast.h>
#include <Common/StringUtils/StringUtils.h>
namespace DB
{
const char * ParserMultiplicativeExpression::operators[] =
{
    "*",     "multiply",
    "/",     "divide",
    "%",     "modulo",
    nullptr
};
const char * ParserUnaryMinusExpression::operators[] =
{
    "-",     "negate",
    nullptr
};