Include :date, :datetime, and :time with minimal options

[eemeli]

See #1082 for some of the discussion leading to this proposal.

This is an alternative to #1077, where instead of dateStyle and timeStyle we start with the following options:

• :datetime
  • dateFields
  • dateLength
  • timePrecision
  • timeZoneStyle
• :date
  • fields
  • length
• :time
  • precision
  • timeZoneStyle

With these, we are able to represent each of the style option combinations rather succinctly:

Style options (#1077)	This PR
:date style=short	:date length=short
:date style=medium	:date length=medium
:date style=long	:date length=long
:date style=full	:date fields=year-month-day-weekday length=long
:time style=short	:time precision=minute
:time style=medium	:time precision=second
:time style=long	:time precision=second timeZoneStyle=short
:time style=full	:time precision=second timeZoneStyle=long
:datetime dateStyle=short timeStyle=medium	:datetime dateLength=short timePrecision=second

For the most part, dateStyle maps to dateLength (and dateFields to bring in the weekday), while timeStyle maps to timePrecision and timeZoneStyle.

This approach allows for the same locale-specific optionality as is currently in CLDR data, with e.g. ps and th including the era with the year, and many locales defaulting to including a dayperiod (am/pm) indicator in the time.

The expectation here is that the options being provided here are sufficiently easy to use and understand without any external reference that we won't need to later introduce dateStyle and timeStyle, while providing a sufficient foundation for filling in the remaining details.

This is not intended to be a complete solution for datetime formatting, but a step towards that. If we accept this, we will need to subsequently and separately consider additional option values like timePrecision=minuteOptional or dateFields=year-month, as well as additional options like yearStyle or fractionalSecondDigits, or even dateStyle and timeStyle. They've all been left out of here to keep this as small as possible, while providing a cohesive whole for many date/time formatting needs.

[sffc]

I'm okay with this, although we should have a discussion with the broader group on the option naming bikeshed.