feat(intl): add ListFormat, DisplayNames, and Segmenter wrappers

[marcstraube]

Summary

Extend the intl module with the remaining Intl wrappers deferred from #139:

• Intl.ListFormat — formatList(items, locale?, options?) ("a, b, and c").
• Intl.DisplayNames — display names for languages/regions/currencies/scripts.
• Intl.Segmenter — grapheme/word/sentence segmentation.

Follow the v1 pattern exactly: cached formatter via getCachedFormatter, throwing

• Result variants, IntlError classification, strict types, 100% coverage,
  mutation >= 85%, and a documentation/intl.md update.

Segmenter has the most nuanced API and can land in a separate step if needed.