react-intl libdef: Add typeof to Intl.PluralRules in some places

This fixes one of the things Flow glosses over and would catch when
we migrate this to a .js.flow file, with an error like this:

  Error -------------------- flow-typed/react-intl_vx.x.x.js:353:36

  Cannot use `PluralRules` as a type. A name can be used as a type
  only if it refers to a type, interface, class, or enum definition.
  To get the type of a non-class value, use `typeof`.
  [value-as-type]

     353|     pluralRules: {| [key: string]: Intl.PluralRules |};
                                             ^^^^^^^^^^^^^^^^

I think this is what's going on:

Flow treats Intl.Foo as a value, because Intl is a `declare var`,
not a `declare type` [1]:

  declare var Intl: {
    Collator: Class<Intl$Collator>,
    DateTimeFormat: Class<Intl$DateTimeFormat>,
    NumberFormat: Class<Intl$NumberFormat>,
    PluralRules: ?Class<Intl$PluralRules>,
    getCanonicalLocales?: (locales?: Intl$Locales) => Intl$Locale[],
    ...
  }

I think that means that if we want to use the declared type of an
Intl.Foo, we'd normally want to use `typeof Intl.Foo`.

But most of the actual Intl members with `Class<…>` are special,
because a class value can be used as a type without `typeof`.

I think Intl.PluralRules doesn't get that special treatment, because
it's got `?Class<…>` -- which expands to the union type
`Class<…> | null | void`. Union types aren't class definitions, so
the special treatment doesn't apply and we should use `typeof`.

[1] https://github.com/facebook/flow/blob/v0.141.0/lib/intl.js#L8-L15

flow-typed/react-intl_vx.x.x.js

@@ -350,7 +350,7 @@ declare module 'react-intl' {
     number: {| [key: string]: Intl.NumberFormat |};
     message: {| [key: string]: IntlMessageFormat |};
     relativeTime: {| [key: string]: RelativeTimeFormat |};
-    pluralRules: {| [key: string]: Intl.PluralRules |};
+    pluralRules: {| [key: string]: typeof Intl.PluralRules |};
     list: {| [key: string]: ListFormat |};
     displayNames: {| [key: string]: DisplayNames |};
   }
@@ -412,7 +412,7 @@ declare module 'react-intl' {
     formatPlural(
       value: number, // Param of Intl.PluralRules.prototype.select()
       opts?: FormatPluralOptions,
-    ): $Call<<R>((...args: any[]) => R) => R, $PropertyType<Intl.PluralRules, 'select'>>;
+    ): $Call<<R>((...args: any[]) => R) => R, $PropertyType<typeof Intl.PluralRules, 'select'>>;
     formatMessage(
       descriptor: MessageDescriptor,
       // The `+` was added to make the properties covariant rather