negative ISO8601 duration strings are not handled properly by the duration function

**Describe the bug**
negative ISO8601 duration strings are not handled properly.
```
const durationString = dayjs.duration(-1000).toISOString(); //Results in -PT1S
expect(durationString).toEqual('-PT1S');
const actual = dayjs.duration(durationString);
expect(actual.asSeconds()).toEqual(-1); // this fails !!! results in +1
```

**Expected behavior**
Solution 1
toISOString should apply negative sign to the numeric portions of the string ( for the above eg it should return PT-1S)

Solution 2
duration should handle leading negative sign as a negative duration. ( for the above example asSeconds should return -1)

**Relates to**
https://github.com/iamkun/dayjs/issues/1700

**Information**
- Day.js Version [e.g. v1.10.7]
- OS: node/ubuntu

No source file or test is named. Start by locating the duration parsing and toISOString entry points, then reproduce the provided -PT1S example; done means negative ISO8601 durations round-trip with asSeconds() returning -1 and regression coverage records the chosen behavior.

Written by the indexing model from the issue text.