Locale: Support for Percent Formatting

Updates BNumberFormat to be able to format
percentages. Also re-introduces some unit
tests and updates the BNumberFormat ones.

This doesn't actually fix #16312 as the
defaults for percentage formatting don't seem
to track the selected language, but goes part
way there.

Related to #16312

Change-Id: Id6ddf426ce5571f4e8513c0eb1663cf42ac53cb1
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3767
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>