Implement localized week information

[diesieben07]

Use Intl.Locale#getWeekInfo to provide locale-dependent week information, such as the start of the week.

Fixes #1447

Current status:

• Info.features reports on whether localized week info is available in the current environment
• Info.getStartOfWeek: returns 1 (Monday) through 7 (Sunday) for first day of the week according to locale
• Info.getMinimumDaysInFirstWeek for getting minimum days in the first week of the year
• Info.getWeekendWeekdays for getting weekend days
• DateTime#isWeekend: returns whether the current date is on the weekend according to the locale
• DateTime#localWeekday: returns 1-7 for first to last day according to locale
• DateTime#localWeekNumber and DateTime#localWeekYear; same as weekNumber and weekYear, but respecting the locale. I need to understand the math here before I can implement it.
• DateTime#hasSame, DateTime#startOf, DateTime#endOf and Interval#count accept a new option useLocaleWeeks, when enabled the week unit will act according to the locale week information
• Tests
• (Potentially) Caching for the localized week data (weekNumber, weekYear)
• Investigate how to let the user override the defaults. There are BCP tags to set the start of the week, but browsers seem to ignore them, so we probably need to provide our own configuration option, similar to numberingSystem.
• Accept locale-based week units in DateTime.fromObject and DateTime#set
• Add DateTime#weeksInLocalWeekYear, analogous to weeksInWeekYear
• Investigate supporting formatting tokens for localWeekNumber and localWeekYear.
• Documentation

[icambron]

Looks really good. Left one comment

[diesieben07]

After a lot of work, this is now ready for review. All features I wanted to add have been implemented, including tests.

[icambron]

This is great. Merging as-is.

[carvalheiro]

Hey! Any Idea when will this be released at npm? @icambron

Amazing work btw 🚀