Custom date/time patterns

[zbraniecki]

One of the most common scenarios is that the user customized their date/time short/medium/long/full pattern in MacOS/Windows preferences.

The decision to use Unicode Extension Keys for conveying the information (you call it "BCP47" which is inaccurate) makes it hard to extend to this use case.

[romulocintra]

Do you have any suggestions other than 'Unicode Extension Keys' ?

[sffc]

I think it may be useful to return data serialized as either text/json or application/x-www-form-urlencoded, whichever is more appropriate in this context.

I do think we should stick with BCP-47 compatibile options, but I think we should spec this out in a way that non-BCP-47 options are able to be supported.