.format('Y') returns timezone offset instead of ISO 8601 year

DayJS is supposed to be fully compatible with MomentJS
```
var d = require('dayjs')
d().format('Y') // returns '+0200' instead of '2021'
```
[Moment docs for .format](https://momentjs.com/docs/#/displaying/format/)
On DayJS's [docs](https://day.js.org/docs/en/display/format) the single `Y` is not documented

Start with DayJS's .format API and compare the single-Y behavior with the linked Moment.js format documentation. Check the existing formatting tests, then verify that .format('Y') returns the ISO 8601 year such as '2021' rather than a timezone offset.

Written by the indexing model from the issue text.