Dates with years with 5 digits do not round-trip

**Describe the bug**
`dayjs(dayjs('20222-02-02').format())` is invalid date.

**Expected behavior**
`dayjs(dayjs('20222-02-02').format())` should be the same as `dayjs('20222-02-02')`.

`dayjs('+020222-02-03T00:00:00.000')` works -- if there's something I can pass to `format` to make it format ISO 8601 if the year is less than 10k, and otherwise like this, that would be a good workaround.

**Information**
- Day.js Version: 1.10.6
- OS: Ubuntu
- Browser: Node
- Time zone: GMT

Start by reproducing the round-trip in Node with dayjs('20222-02-02').format() and the resulting parser call. Trace the format and parse entry points for extended years, then verify that formatting and reparsing produce the same date, including the existing +020222 ISO form.

Written by the indexing model from the issue text.