[Pitch] Support notation when formatting currencies

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Hi everyone,

I have a simple pitch for supporting notation for Currency FormatStyles.

Support notation when formatting currencies

Introduction/Motivation

When formatting numbers or percents, we can specify a notation, for example:

let count = 1_500_000
let result = count.formatted(.number.notation(.compactName))
assert(result == "1.5M")

We will enable this for currency formatters as well.

Proposed solution

Add the notation modifier to the various currency formatters. All of the notations supported by the number and percent formatters will be supported by the currency formatters.

let price = Decimal(1_500_000.59)
let result = price.formatted(.currency(code: "USD").notation(.compactName))
assert(result == "$1.5M")

The underlying ICU formatters already support this, so we just have to forward the appropriate options.

Detailed design

extension IntegerFormatStyle.Currency {
    /// Modifies the format style to use the specified notation.
    ///
    /// - Parameter notation: The notation to apply to the format style.
    /// - Returns: An integer currency format style modified to use the specified notation.
    @available(FoundationPreview 0.4, *)
    public func notation(_ notation: Configuration.Notation) -> Self
}

extension FloatingPointFormatStyle.Currency {
    /// Modifies the format style to use the specified notation.
    ///
    /// - Parameter notation: The notation to apply to the format style.
    /// - Returns: A floating-point currency format style modified to use the specified notation.
    @available(FoundationPreview 0.4, *)
    public func notation(_ notation: Configuration.Notation) -> Self
}

extension Decimal.FormatStyle.Currency {
    /// Modifies the format style to use the specified notation.
    ///
    /// - Parameter notation: The notation to apply to the format style.
    /// - Returns: A decimal currency format style modified to use the specified notation.
    @available(FoundationPreview 0.4, *)
    public func notation(_ notation: Configuration.Notation) -> Self
}

extension CurrencyFormatStyleConfiguration {
    /// The type used to configure notation for currency format styles.
    @available(FoundationPreview 0.4, *)
    public typealias Notation = NumberFormatStyleConfiguration.Notation
}

Source compatibility

These changes are additive only and are not expected to have an impact on source compatibility.

Implications on adoption

This new API will have FoundationPreview 0.4 availability.

Alternatives considered

None.

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