fix: keep sign when zero-padding negative years in format('YYYY') (#2722)

[anas510]

Fixes #2722

padStart counted the minus sign as a character to pad around, so negative (BC) years rendered wrongly:

• dayjs('-000001').format('YYYY') → 00-1 (now -0001)

• dayjs('-000010').format('YYYY') → 0-10 (now -0010)

• dayjs('-000100').format('YYYY') → -100 (now -0100)

• padStart now pads the absolute value and re-applies the sign, as Moment's zeroFill does.

• No change for non-negative inputs; duration().format() with negative components benefits the same way (0-1 → -01).

• Bundle size +~20 B (2.81 KB, under the 2.99 KB limit).

Out of scope: years 0–99 are still shifted by new Date(y, m, d) (#2591); YY for negative years is unchanged.