Offset bug with non 4 digit input dates

File:
src/offset.ts

Bug:
The offset function returns `+NaN:NaN` when called with non 4 digit year date input like

```typescript
offset('0001-01-01')
```

This is because the `Intl.DateTimeFormat.prototype.formatToParts`  function being used in line 24 does not pad the year number.

Solution:
Add padding for the year in line 38