This uses ICU for the Intl.DateTimeFormat `format` `formatToParts`,
`formatRange`, and `formatRangeToParts`.
This lets us remove most data from our date-time format generator. All
that remains are time zone data and locale week info, which are relied
upon still for other interfaces. So they will be removed in a future
patch.
Note: All of the changes to the test files in this patch are now aligned
with other browsers. This includes:
* Some very incorrect formatting of Japanese symbols. (Looking at the
old results now, it's very obvious they were wrong.)
* Old FIXMEs regarding range formatting not including the start/end date
when only time fields were requested, but the dates differ.
* Day period inconsistencies.
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