To prepare for using plural rules within number & duration format, this
removes the NumberFormat::Plurality enumeration.
This also adds PluralCategory::ExactlyZero & PluralCategory::ExactlyOne.
These are used in locales like French, where PluralCategory::One really
means any value from 0.00 to 1.99. PluralCategory::ExactlyOne means only
the value 1, as the name implies. These exact rules are not known by the
general plural rules, they are explicitly for number / currency format.
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