Use locale for DateTimeFormat timezone names

[s-summers]

Currently the DateTimeFormat that is created uses en-US as the locale. However this is not sufficent to include the desired timezone name.

dayjs/src/plugin/timezone/index.js

Lines 15 to 34 in 1d8746c

	const getDateTimeFormat = (timezone, options = {}) => {
	const timeZoneName = options.timeZoneName || 'short'
	const key = `${timezone}|${timeZoneName}`
	let dtf = dtfCache[key]
	if (!dtf) {
	dtf = new Intl.DateTimeFormat('en-US', {
	hour12: false,
	timeZone: timezone,
	year: 'numeric',
	month: '2-digit',
	day: '2-digit',
	hour: '2-digit',
	minute: '2-digit',
	second: '2-digit',
	timeZoneName
	})
	dtfCache[key] = dtf
	}
	return dtf
	}

For example:

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

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.