WeekInfoOfLocale: Handle "ca" Unicode extension

[anba]

WeekInfoOfLocale should handle the ca Unicode extension per First Day Overrides.

See also #1051 which adds RegionPreference to WeekInfoOfLocale.

(There's currently only a note referencing UTS 35 "First Day Overrides", but notes aren't normative.)

[ptomato]

Does this make sense with how we consider calendars in ECMAScript? "First Day Overrides" says

if there is a valid -u-ca- calendar value, where that calendar specifies the first day, then return that first day. (Most calendars do not specify the first day.)

I'm looking at ICU4C's Calendar::getFirstDayOfWeek() method, and the documentation says

Gets what the first day of the week is; e.g., Sunday in US, Monday in France.

Certainly the concept of a calendar in Temporal is not associated with a region, so if you have Intl.Locale('en-us-u-ca-gregory') and Intl.Locale('fr-fr-u-ca-gregory') any difference in the first day of the week is going to come from us or fr, not from gregory.

For locales that use more than one calendar e.g.

> new Intl.Locale('bn').getCalendars()
// => ['gregory', 'islamic-civil', 'islamic-tbla']

Is it ever the case that you'd want a different first day depending on whether your locale was bn-u-ca-gregory or bn-u-ca-islamic-civil? I assume it might be, but I was looking in CLDR to see if this was in fact specified anywhere and I wasn't able to find any conclusive evidence one way or the other. If anyone else knows where to find this, please share!

[sffc]

It's about giving hints and allowing the calendar to influence this even if it doesn't do so currently.

For instance, en-US-u-ca-iso8601 would very reasonably return Monday.