Add support for more currency symbol variants (e.g. formal)

[sffc]

Currently we support four currencyDisplay values:

• "symbol" to use a localized currency symbol such as €, this is the default value,
• "narrowSymbol" to use a narrow format symbol ("$100" rather than "US$100"),
• "code" to use the ISO currency code,
• "name" to use a localized currency name such as "dollar",

However, CLDR specifies several additional styles. We should consider adding these to ECMA-402.

Feature request for the "formal" style: #642