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

[kaujaj]

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
#1700

Information

• Day.js Version [e.g. v1.10.7]
• OS: node/ubuntu