Handle multiple lengths of the same field in DateTimeNames

[sffc]

With the date symbols redesign (#3865) and the new DateTimePatternInterpolator (#3347), we now support one length of each symbol during formatting. For example, we don't support a pattern with both MMM and MMMM at the same time.

We could relax this in the future.

[sffc]

Would like to discuss with:

• @Manishearth
• @zbraniecki
• @eggrobin
• @sffc

[sffc]

Something worth considering is whether we want to use LiteMap<Field, DataNamesPayload, ShortVec>, which reduces stack space but requires an allocation for patterns containing more than 1 field with display names. This covers a number of common cases, such as "9:48 pm" or "Dec 26, 2023" or "12/26/2023 at 9:48 pm", but it puts other cases on a slow path.

[sffc]

• @eggrobin - If you ask for a time zone or a weekday, a time zone or a weekday could appear at any length. Once you resolve a particular pattern in a locale, it would supprise me somewhat if multiple lengths appeared in the same pattern for the same field.
• @sffc - How about across plural forms?
• @eggrobin - That would surprise me but surprise me less.

Conclusion: If we find we need to support this, we don't need a particularly efficient solution. The common case of a single length per field should be efficient.

LGTM: @eggrobin @sffc @Manishearth

Actual implementation: likely use something similar to ShortBoxSlice or DataPayloadOr.