Add support for various ICU features

Thanks for your work on this. As I work extensively with text rendering I know how gnarly all of this is.

For my work on https://github.com/r-lib/textshaping I need access to a few ICU features that aren't currently exposed by stringi and I'd rather not have to link to ICU myself, so I was wondering if you were open to adding these to stringi?

I'm currently looking especially to get the script associated with a given unicode point, whether the glyph/character is considered full-width, as well as formatting numbers in different scripts (e.g. 45 -> 四十五).

Is such functionality something you'd consider adding?

Start by reviewing stringi's existing ICU-related entry points and the three requested capabilities: Unicode script lookup, full-width character detection, and number formatting across scripts. The issue names no files or tests, so first establish the intended API scope and confirm what completion should cover with the maintainers.

Written by the indexing model from the issue text.