Add #as_skeleton and #set_date_format_from_skeleton to DateTimeFormatter

[KJTsanaktsidis]

As documented in the tests, the main purpose of this functionality is to allow:

• Loading up one of the standard date/time patterns :long, :short, etc
• Converting it into a skeleton
• Manipulating the skeleton somehow
• Converting the skeleton back into a real pattern
• Formatting a time with it

The use-case in question I have is being able to modify the standard skeleton patterns to use j for locale-appropriate 12/24 hour time, instead of H/h being specified per-locale. This means that locales with hours keywords like "en_AU@hours=h23" can be made to produce 24 hour time, which ICU would not normally do.

[lwelti]

I think is relevant to confirm your use case. Input: date, Locale, options Output?

[KJTsanaktsidis]

We discussed this today in our meeting - I don't think this is an especially good idea 😂