In skeleton matching, combine date and time patterns

[gregtatum]

https://unicode.org/reports/tr35/tr35-dates.html#Matching_Skeletons

1. Divide the request into a date fields part and a time fields part.
2. For each part, find the matching dateFormatItem, and expand the pattern as above.
3. Combine the patterns for the two dateFormatItems using the appropriate dateTimeFormat pattern, determined as follows from the requested date fields:

• If the requested date fields include wide month (MMMM, LLLL) and weekday name of any length (e.g. E, EEEE, c, cccc), use
• Otherwise, if the requested date fields include wide month, use
• Otherwise, if the requested date fields include abbreviated month (MMM, LLL), use
• Otherwise use

This is a follow-up to #481. It blocks #272.

[gregtatum]

2020-04-02: This needs to be implemented.

My current status on this is that I have a working copy of this locally.