Add nice currency formatting support

[Swatinem]

The support for currency formatting in messageformat is lacking :-( https://formatjs.io/guides/message-syntax/#custom-formats

I propose the following special custom formats (assuming locale de):

• support for either hardcode a currency or passing one as parameter: currency:EUR, currency:$code
• support for formatting a omitting decimals when possible: currency0
• support for using the code instead of the default symbol: currencycode

All permutations of these should be supported, for example currencycode0:$code

Examples:

• {value, number, currency:EUR} will result in 10,00 €
• {value, number, currency:$code} will result in 10 ¥ when passing { code: 'JPY' }
• {value, number, currency0:EUR} will result in 10 €
• {value, number, currencycode:EUR} will result in 10,00 EUR

[Swatinem]

Now that I thought more about this, and also looked at how fluent does currency formatting, I think it would be a good idea to just introduce a proper currency type, which is defined as {value: number, code: ISO4217 | string} and always define the currency via code instead of via the translation.

So what is left would be:

declare const price: { value: number, code: string }
const translations = [
  "{price, number, currency}",
  "{price, number, currency0}",
  "{price, number, currencycode}",
]