[Pitch] Default values for string interpolations

[Nevin]

@nnnnnnnn

Instead of extending String.StringInterpolation, we could extend StringInterpolationProtocol where StringLiteralType == String. That will also provide the same functionality to LocalizedStringKey, though I don't know if that would require additional work inside the defining framework.

Is there official guidance on where such string interpolations should be defined?

When I asked about creating a "\(n, radix: r)" interpolation a few years ago, one of the replies said:

One of the main reasons to create and use a conformer besides DefaultStringInterpolation is in order to restrict the available interpolations at compile time as a correctness aid in a particular situation. By extending every such type wholesale, you may be shooting yourself in the foot.