SE-0211: Add Unicode Properties to Unicode.Scalar

[allevato (Tony Allevato)]

@AliSoftware in the companion pitch thread for this proposal had an excellent suggestion:

Love this idea. Just my two cents, I’d make the Unicode.GeneralCategory enum RawRepresentable (enum Unicode.GeneralCategory: String) so that we can access the two-letter codes if needed

This could especially be useful for readability and clarity when creating regular expressions referencing those character categories, using "\p{(Unicode.GeneralCategory.lowercaseLetter)}" for example.

Adding this would be a trivial change and I think it would be an excellent enhancement to the API.