The PluralCategory enum is currently generated for plural rules. Instead
of generating it, this moves the enum to the public LibUnicode header.
While it was nice to auto-discover these values, they are well defined
by TR-35, and we will need their values from within the number format
code generator (which can't rely on the plural rules generator having
run yet). Further, number format will require additional values in the
enum that plural rules doesn't know about.
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.
