Consider support for millisecond field

[sffc]

In datetime patterns, we currently support second (s) and fractional second (S). This issue is to track potentially adding support for the millisecond field (A).

References:

• Implement formatting for fractional seconds #1813 (review)

• icu4x/components/datetime/src/format/datetime.rs

  Line 321 in 1199f96

  field @ FieldSymbol::Second(Second::Millisecond) => {