Use Info.Locale.weekInfo to determine start of week

[JapNolt]

It appears like Intl.Locale.weekInfo is implemented in most browsers. Can this be used in luxon to determine the correct start of week?

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/Locale/weekInfo

[diesieben07]

Generally a good idea, however a few things to keep in mind:

• 86.4% is a big chunk of browsers, but hardly covers enough for Luxon to just use this API without fallback.
• Currently startOf('week') in Luxon means Monday, no matter the locale. As such there would need to be a separate way to get the locale dependent start of the week. This is also because sometimes you do not want the locale dependent start of the week.
• There probably needs to be a way to override the start of week, regardless of what the locale data says, similar to numberingSystem or outputCalendar.
• There is a BCP 47 extension for this (e.g. en-US-u-fw-wed to specify Wednesday as the start of the week), however this seems to be ignored by the Intl.Locale API, it always provides the data of the locale. Luxon doesn't have a BCP-47 parser, but if a DateTime's locale contains this tag, we probably want to honor it.