[react-i18n] Handle explicit "0" and "1" pluralization keys

Overview

As described in the Common Locale Data Repository (CLDR), add support for explicit "0" and "1" pluralization keys.

The explicit “0” and “1” cases are not defined by language-specific rules, and are available in any language where the content provides them.
The explicit “0” and “1” cases apply to the exact numeric values 0 and 1 respectively. These cases are typically used for plurals of items that do not have fractional value, like books or files.
The explicit “0” and “1” cases have precedence over the “zero” and “one” cases. For example, if for a particular element CLDR data includes values for both the “1” and “one” cases, then the “1” value is used for numeric values of exactly 1, while the “one” value is used for any other formatted numeric values matching the “one” plural rule for the language.
The key names should be strings, not integers.

Usage example: In English (which only defines language-specific rules for “one” and “other”) this can be used to have special behavior for 0:

Typically, this is achieved by having completely separate keys for the explicit "0" and "1" use cases:

{
  "zeroProducts": "No products",
  "products": {
    "one": "{count} product",
    "other": "{count} products",
  }
}

This would then have to be handled explicitly in code:

const label = products.length === 0 ? i18n.translate('zeroProducts') : i18n.t('products', { count: products.length });

Once the explicit handling of the "0" and "1" case is complete, the key definition becomes simpler:

{
  "products": {
    "0": "No products",
    "one": "{count} product",
    "other": "{count} products",
  }
}

The implementation in code also becomes simpler:

const label = i18n.t('products', { count: products.length });

What inspired this feature request? What problems were you facing, or what else makes you think this should be in quilt?

Provide simpler implementation of common pluralization use cases.