feature request - defaultFormat and defaultFormatUtc

Hi! I'm migrating some `moment-timezone` code to `dayjs`.

There's one thing I can't figure out how to set in `dayjs`:

```js
moment.defaultFormat = 'YYYY-MM-DDTHH:mm:ss.SSS[Z]';
moment.defaultFormatUtc = 'YYYY-MM-DDTHH:mm:ss.SSS[Z]';
```

I couldn't find anything similar in GitHub issues or in the documentation.

Any help is appreciated!

Start by locating how Day.js currently handles default date and UTC formatting, then compare that behavior with the requested Moment.js settings. Done means users can configure both default formats and receive the requested output consistently.

Written by the indexing model from the issue text.