dayjs constructor does not work with strict flag set to true.

**Describe the bug**
```
const format = 'YYYY-MM-DDTHH:mm:ss.SSSZ';
const inputDate = '1993-12-01T10:22:33.128Z';
const isDateValid = dayjs(inputDate, format, true).isValid(); // returns false
```
another example
```
dayjs("2021-01-26T14:25:04.871Z", "YYYY-MM-DDTHH:mm:ss.SSSZ", true) //doesn't work - returns invalid Date
moment("2021-01-26T14:25:04.871Z", "YYYY-MM-DDTHH:mm:ss.SSSZ",true) //works
```
please see https://github.com/iamkun/dayjs/issues/1358#issuecomment-774296660

**Expected behavior**
In the first example, isValid() should return true
In the second example, dayjs should behave in the same manner as moment.

**Information**
- Day.js Version 1.10.7
- OS: ubuntu nodejs
- Time zone: Central Daylight Time -06:00

Start by running the reported dayjs constructor examples with the strict flag and compare their results with Moment. Trace the constructor's formatted-date parsing path, then add coverage for the ISO timestamp cases; done means both examples validate successfully and match the stated Moment behavior.

Written by the indexing model from the issue text.