Support leap months in `datetime`

[atcupps]

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Currently, #3760 supports leap months with a placeholder by writing (leap) before a leap month because the infrastructure to generate leap month data and include it in formatted datetimes does not exist. This is similar to but separate from #3761. I discussed this with @sffc and @Manishearth and they formed a plan to address this.

[Manishearth]

Plan for this for 1.3 is to implement hardcoded mostly-correct behavior

https://github.com/unicode-org/icu4x/issues/3865#issuecomment-1692219467

[Manishearth]

This is supported in neo.