Use locale for DateTimeFormat timezone names

Currently the `DateTimeFormat` that is created uses `en-US` as the locale. However this is not sufficent to include the desired timezone name.
https://github.com/iamkun/dayjs/blob/1d8746c23bd667bde80ee627a915301ebd69e1a2/src/plugin/timezone/index.js#L15-L34

For example:
```ts
getDateTimeFormat('Australia/Sydney').format(new Date())
// '09/15/2025, 09:39:52 GMT+10'
```
whereas if it was switched to `en-AU` we get the following
```ts
getDateTimeFormat('Australia/Sydney').format(new Date())
// '15/09/2025, 09:39:52 AEST'
```
Notice that with `en-US` the timezone is `GMT+10` but with `en-AU` it is `AEST`.

I would like to request a change such that the `dayjs.locale` set by imports like `import 'dayjs/locale/en-au'` is also used when constructing the `DateTimeFormat`.

Start in src/plugin/timezone/index.js at the linked getDateTimeFormat implementation and inspect how the active dayjs locale is exposed. Verify the formatter's timezone-name output for Australia/Sydney with en-US and en-AU; done means the configured dayjs locale is used and the expected localized timezone name is produced.

Written by the indexing model from the issue text.