fix: parse datetime64 precision with timezone

[surasithaof]

DateTime64 can have timezone. If the column defined with the timezone for example DateTime64(3, 'UTC'), When query try to parse value, the precision will always return as 0 because it errors.

In the parseDateTime64Prec function, when it passed chSubType(s, "DateTime64(") will got the value like "3, 'UTC" and will always errors when parse string to integer.

So we need to split by comma and take the first part to get the precision before convert to integer.