The NumberFormat spec casually indicates the need for a PluralRules
object without explicity saying so, with text such as:
"which may depend on x in languages having different plural forms."
Other implementations actually do create a PluralRules object to resolve
those cases with ResolvePlural. However, ResolvePlural doesn't need much
from PluralRules to operate, so this can be abstracted out for use in
NumberFormat without the need to allocate a PluralRules instance.
The JS::Intl enum was added when implementing the PluralRules
constructor. Now that LibUnicode has a plural rules implementation,
replace the JS::Intl enum with the analagous Unicode enum.
