fix: iso8601 week start and use intl.locale when possible for week start

[snowystinger]

Closes #8810

Running new Intl.Locale('en-US-u-ca-iso8601').getWeekInfo() in supported browsers does return Monday
iso8601 means that the region does not matter https://www.unicode.org/reports/tr35/tr35-72/tr35-dates.html#Week_Data

Technically, the first day is also overridable via fw-tue for example. I've included some parsing for that in browsers that don't support Intl.Locale yet. Is this something we want?

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
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• Looked at the Accessibility Practices for this feature - Aria Practices

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[LFDanLu]

Verified the behavior on browsers that support getWeekInfo (Chrome/Safari) and that don't (Firefox). I think its fine to add the -fw- logic for now, hopefully https://bugzilla.mozilla.org/show_bug.cgi?id=1693576 which includes this support in FF gets added soon

[snowystinger]

@yihuiliao @LFDanLu updated and fixed merge, thank you!

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